diff --git a/src/ap/ctrl_iface_ap.c b/src/ap/ctrl_iface_ap.c
index 976093342..ccbbab5b0 100644
--- a/src/ap/ctrl_iface_ap.c
+++ b/src/ap/ctrl_iface_ap.c
@@ -405,6 +405,7 @@ int hostapd_ctrl_iface_status(struct hostapd_data *hapd, char *buf,
 			  "num_sta_ht_no_gf=%d\n"
 			  "num_sta_no_ht=%d\n"
 			  "num_sta_ht_20_mhz=%d\n"
+			  "num_sta_ht40_intolerant=%d\n"
 			  "olbc_ht=%d\n"
 			  "ht_op_mode=0x%x\n",
 			  hostapd_state_text(iface->state),
@@ -417,6 +418,7 @@ int hostapd_ctrl_iface_status(struct hostapd_data *hapd, char *buf,
 			  iface->num_sta_ht_no_gf,
 			  iface->num_sta_no_ht,
 			  iface->num_sta_ht_20mhz,
+			  iface->num_sta_ht40_intolerant,
 			  iface->olbc_ht,
 			  iface->ht_op_mode);
 	if (ret < 0 || (size_t) ret >= buflen - len)
diff --git a/src/ap/drv_callbacks.c b/src/ap/drv_callbacks.c
index a8c24ebdf..b3cfade9f 100644
--- a/src/ap/drv_callbacks.c
+++ b/src/ap/drv_callbacks.c
@@ -9,6 +9,7 @@
 #include "utils/includes.h"
 
 #include "utils/common.h"
+#include "utils/eloop.h"
 #include "radius/radius.h"
 #include "drivers/driver.h"
 #include "common/ieee802_11_defs.h"
@@ -30,6 +31,7 @@
 #include "ap_config.h"
 #include "hw_features.h"
 #include "dfs.h"
+#include "beacon.h"
 
 
 int hostapd_notif_assoc(struct hostapd_data *hapd, const u8 *addr,
@@ -121,6 +123,24 @@ int hostapd_notif_assoc(struct hostapd_data *hapd, const u8 *addr,
 	}
 #endif /* CONFIG_P2P */
 
+#ifdef CONFIG_IEEE80211N
+#ifdef NEED_AP_MLME
+	if (elems.ht_capabilities &&
+	    elems.ht_capabilities_len >=
+	    sizeof(struct ieee80211_ht_capabilities) &&
+	    (hapd->iface->conf->ht_capab &
+	     HT_CAP_INFO_SUPP_CHANNEL_WIDTH_SET)) {
+		struct ieee80211_ht_capabilities *ht_cap =
+			(struct ieee80211_ht_capabilities *)
+			elems.ht_capabilities;
+
+		if (le_to_host16(ht_cap->ht_capabilities_info) &
+		    HT_CAP_INFO_40MHZ_INTOLERANT)
+			ht40_intolerant_add(hapd->iface, sta);
+	}
+#endif /* NEED_AP_MLME */
+#endif /* CONFIG_IEEE80211N */
+
 #ifdef CONFIG_INTERWORKING
 	if (elems.ext_capab && elems.ext_capab_len > 4) {
 		if (elems.ext_capab[4] & 0x01)
diff --git a/src/ap/hostapd.c b/src/ap/hostapd.c
index 614a5bf40..33988598a 100644
--- a/src/ap/hostapd.c
+++ b/src/ap/hostapd.c
@@ -33,6 +33,7 @@
 #include "p2p_hostapd.h"
 #include "gas_serv.h"
 #include "dfs.h"
+#include "ieee802_11.h"
 
 
 static int hostapd_flush_old_stations(struct hostapd_data *hapd, u16 reason);
@@ -1352,6 +1353,11 @@ void hostapd_interface_deinit(struct hostapd_iface *iface)
 	if (iface == NULL)
 		return;
 
+#ifdef CONFIG_IEEE80211N
+#ifdef NEED_AP_MLME
+	eloop_cancel_timeout(ap_ht2040_timeout, iface, NULL);
+#endif /* NEED_AP_MLME */
+#endif /* CONFIG_IEEE80211N */
 	eloop_cancel_timeout(channel_list_update_timeout, iface, NULL);
 	iface->wait_channel_update = 0;
 
diff --git a/src/ap/hostapd.h b/src/ap/hostapd.h
index 090544d5a..dba9bb29c 100644
--- a/src/ap/hostapd.h
+++ b/src/ap/hostapd.h
@@ -327,6 +327,9 @@ struct hostapd_iface {
 	/* Number of HT associated stations 20 MHz */
 	int num_sta_ht_20mhz;
 
+	/* Number of HT40 intolerant stations */
+	int num_sta_ht40_intolerant;
+
 	/* Overlapping BSS information */
 	int olbc_ht;
 
@@ -351,6 +354,10 @@ struct hostapd_iface {
 	unsigned int dfs_cac_ms;
 	struct os_reltime dfs_cac_start;
 
+	/* Latched with the actual secondary channel information and will be
+	 * used while juggling between HT20 and HT40 modes. */
+	int secondary_ch;
+
 #ifdef CONFIG_ACS
 	unsigned int acs_num_completed_scans;
 #endif /* CONFIG_ACS */
diff --git a/src/ap/hw_features.c b/src/ap/hw_features.c
index d47a366b8..43cb243b9 100644
--- a/src/ap/hw_features.c
+++ b/src/ap/hw_features.c
@@ -19,6 +19,8 @@
 #include "ap_config.h"
 #include "ap_drv_ops.h"
 #include "acs.h"
+#include "ieee802_11.h"
+#include "beacon.h"
 #include "hw_features.h"
 
 
@@ -414,6 +416,7 @@ static int ieee80211n_check_40mhz_2g4(struct hostapd_iface *iface,
 		int pri = bss->freq;
 		int sec = pri;
 		int sec_chan, pri_chan;
+		struct ieee802_11_elems elems;
 
 		ieee80211n_get_pri_sec_chan(bss, &pri_chan, &sec_chan);
 
@@ -445,7 +448,23 @@ static int ieee80211n_check_40mhz_2g4(struct hostapd_iface *iface,
 			}
 		}
 
-		/* TODO: 40 MHz intolerant */
+		ieee802_11_parse_elems((u8 *) (bss + 1), bss->ie_len, &elems,
+				       0);
+		if (elems.ht_capabilities &&
+		    elems.ht_capabilities_len >=
+		    sizeof(struct ieee80211_ht_capabilities)) {
+			struct ieee80211_ht_capabilities *ht_cap =
+				(struct ieee80211_ht_capabilities *)
+				elems.ht_capabilities;
+
+			if (le_to_host16(ht_cap->ht_capabilities_info) &
+			    HT_CAP_INFO_40MHZ_INTOLERANT) {
+				wpa_printf(MSG_DEBUG,
+					   "40 MHz Intolerant is set on channel %d in BSS "
+					   MACSTR, pri, MAC2STR(bss->bssid));
+				return 0;
+			}
+		}
 	}
 
 	return 1;
@@ -475,6 +494,7 @@ static void ieee80211n_check_scan(struct hostapd_iface *iface)
 		oper40 = ieee80211n_check_40mhz_2g4(iface, scan_res);
 	wpa_scan_results_free(scan_res);
 
+	iface->secondary_ch = iface->conf->secondary_channel;
 	if (!oper40) {
 		wpa_printf(MSG_INFO, "20/40 MHz operation not permitted on "
 			   "channel pri=%d sec=%d based on overlapping BSSes",
@@ -482,9 +502,21 @@ static void ieee80211n_check_scan(struct hostapd_iface *iface)
 			   iface->conf->channel +
 			   iface->conf->secondary_channel * 4);
 		iface->conf->secondary_channel = 0;
+		if (iface->drv_flags & WPA_DRIVER_FLAGS_HT_2040_COEX) {
+			/*
+			 * TODO: Could consider scheduling another scan to check
+			 * if channel width can be changed if no coex reports
+			 * are received from associating stations.
+			 */
+		}
 	}
 
 	res = ieee80211n_allowed_ht40_channel_pair(iface);
+	if (!res) {
+		iface->conf->secondary_channel = 0;
+		wpa_printf(MSG_INFO, "Fallback to 20 MHz");
+	}
+
 	hostapd_setup_interface_complete(iface, !res);
 }
 
diff --git a/src/ap/ieee802_11.h b/src/ap/ieee802_11.h
index 809b4ca6c..5b9437151 100644
--- a/src/ap/ieee802_11.h
+++ b/src/ap/ieee802_11.h
@@ -39,6 +39,7 @@ static inline int ieee802_11_get_mib_sta(struct hostapd_data *hapd,
 #endif /* NEED_AP_MLME */
 u16 hostapd_own_capab_info(struct hostapd_data *hapd, struct sta_info *sta,
 			   int probe);
+void ap_ht2040_timeout(void *eloop_data, void *user_data);
 u8 * hostapd_eid_ext_capab(struct hostapd_data *hapd, u8 *eid);
 u8 * hostapd_eid_qos_map_set(struct hostapd_data *hapd, u8 *eid);
 u8 * hostapd_eid_supp_rates(struct hostapd_data *hapd, u8 *eid);
@@ -59,6 +60,8 @@ void hostapd_get_vht_capab(struct hostapd_data *hapd,
 u16 copy_sta_ht_capab(struct hostapd_data *hapd, struct sta_info *sta,
 		      const u8 *ht_capab, size_t ht_capab_len);
 void update_ht_state(struct hostapd_data *hapd, struct sta_info *sta);
+void ht40_intolerant_add(struct hostapd_iface *iface, struct sta_info *sta);
+void ht40_intolerant_remove(struct hostapd_iface *iface, struct sta_info *sta);
 u16 copy_sta_vht_capab(struct hostapd_data *hapd, struct sta_info *sta,
 		       const u8 *vht_capab, size_t vht_capab_len);
 u16 set_sta_vht_opmode(struct hostapd_data *hapd, struct sta_info *sta,
diff --git a/src/ap/ieee802_11_ht.c b/src/ap/ieee802_11_ht.c
index 1d64748fc..38caaed27 100644
--- a/src/ap/ieee802_11_ht.c
+++ b/src/ap/ieee802_11_ht.c
@@ -10,12 +10,15 @@
 #include "utils/includes.h"
 
 #include "utils/common.h"
+#include "utils/eloop.h"
 #include "common/ieee802_11_defs.h"
 #include "hostapd.h"
 #include "ap_config.h"
 #include "sta_info.h"
 #include "beacon.h"
 #include "ieee802_11.h"
+#include "hw_features.h"
+#include "ap_drv_ops.h"
 
 
 u8 * hostapd_eid_ht_capabilities(struct hostapd_data *hapd, u8 *eid)
@@ -200,6 +203,52 @@ u16 copy_sta_ht_capab(struct hostapd_data *hapd, struct sta_info *sta,
 }
 
 
+void ht40_intolerant_add(struct hostapd_iface *iface, struct sta_info *sta)
+{
+	if (iface->current_mode->mode != HOSTAPD_MODE_IEEE80211G)
+		return;
+
+	wpa_printf(MSG_INFO, "HT: Forty MHz Intolerant is set by STA " MACSTR
+		   " in Association Request", MAC2STR(sta->addr));
+
+	if (sta->ht40_intolerant_set)
+		return;
+
+	sta->ht40_intolerant_set = 1;
+	iface->num_sta_ht40_intolerant++;
+	eloop_cancel_timeout(ap_ht2040_timeout, iface, NULL);
+
+	if (iface->conf->secondary_channel &&
+	    (iface->drv_flags & WPA_DRIVER_FLAGS_HT_2040_COEX)) {
+		iface->conf->secondary_channel = 0;
+		ieee802_11_set_beacons(iface);
+	}
+}
+
+
+void ht40_intolerant_remove(struct hostapd_iface *iface, struct sta_info *sta)
+{
+	if (!sta->ht40_intolerant_set)
+		return;
+
+	sta->ht40_intolerant_set = 0;
+	iface->num_sta_ht40_intolerant--;
+
+	if (iface->num_sta_ht40_intolerant == 0 &&
+	    (iface->conf->ht_capab & HT_CAP_INFO_SUPP_CHANNEL_WIDTH_SET) &&
+	    (iface->drv_flags & WPA_DRIVER_FLAGS_HT_2040_COEX)) {
+		unsigned int delay_time = OVERLAPPING_BSS_TRANS_DELAY_FACTOR *
+			iface->conf->obss_interval;
+		wpa_printf(MSG_DEBUG,
+			   "HT: Start 20->40 MHz transition timer (%d seconds)",
+			   delay_time);
+		eloop_cancel_timeout(ap_ht2040_timeout, iface, NULL);
+		eloop_register_timeout(delay_time, 0, ap_ht2040_timeout,
+				       iface, NULL);
+	}
+}
+
+
 static void update_sta_ht(struct hostapd_data *hapd, struct sta_info *sta)
 {
 	u16 ht_capab;
@@ -227,6 +276,9 @@ static void update_sta_ht(struct hostapd_data *hapd, struct sta_info *sta)
 			   __func__, MAC2STR(sta->addr),
 			   hapd->iface->num_sta_ht_20mhz);
 	}
+
+	if (ht_capab & HT_CAP_INFO_40MHZ_INTOLERANT)
+		ht40_intolerant_add(hapd->iface, sta);
 }
 
 
@@ -288,3 +340,14 @@ void hostapd_get_ht_capab(struct hostapd_data *hapd,
 
 	neg_ht_cap->ht_capabilities_info = host_to_le16(cap);
 }
+
+
+void ap_ht2040_timeout(void *eloop_data, void *user_data)
+{
+	struct hostapd_iface *iface = eloop_data;
+
+	wpa_printf(MSG_INFO, "Switching to 40 MHz operation");
+
+	iface->conf->secondary_channel = iface->secondary_ch;
+	ieee802_11_set_beacons(iface);
+}
diff --git a/src/ap/sta_info.c b/src/ap/sta_info.c
index f5417de0c..60f07682e 100644
--- a/src/ap/sta_info.c
+++ b/src/ap/sta_info.c
@@ -206,6 +206,10 @@ void ap_free_sta(struct hostapd_data *hapd, struct sta_info *sta)
 		hapd->iface->num_sta_ht_20mhz--;
 	}
 
+#ifdef CONFIG_IEEE80211N
+	ht40_intolerant_remove(hapd->iface, sta);
+#endif /* CONFIG_IEEE80211N */
+
 #ifdef CONFIG_P2P
 	if (sta->no_p2p_set) {
 		sta->no_p2p_set = 0;
diff --git a/src/ap/sta_info.h b/src/ap/sta_info.h
index 2dbdeb181..03db98f66 100644
--- a/src/ap/sta_info.h
+++ b/src/ap/sta_info.h
@@ -54,6 +54,7 @@ struct sta_info {
 	unsigned int no_short_preamble_set:1;
 	unsigned int no_ht_gf_set:1;
 	unsigned int no_ht_set:1;
+	unsigned int ht40_intolerant_set:1;
 	unsigned int ht_20mhz_set:1;
 	unsigned int no_p2p_set:1;
 	unsigned int qos_map_enabled:1;
diff --git a/src/common/ieee802_11_defs.h b/src/common/ieee802_11_defs.h
index cb70130d3..8fe2e4a67 100644
--- a/src/common/ieee802_11_defs.h
+++ b/src/common/ieee802_11_defs.h
@@ -646,6 +646,7 @@ struct ieee80211_vht_operation {
 #define ERP_INFO_USE_PROTECTION BIT(1)
 #define ERP_INFO_BARKER_PREAMBLE_MODE BIT(2)
 
+#define OVERLAPPING_BSS_TRANS_DELAY_FACTOR 5
 
 /* HT Capabilities Info field within HT Capabilities element */
 #define HT_CAP_INFO_LDPC_CODING_CAP		((u16) BIT(0))