According to the CEO of Espressif, Teo Swee Ann, the I/O pins of the ESP8266 are 5V tolerant. See discussion here: https://hackaday.com/2016/07/28/ask-hackaday

According to the CEO of Espressif, Teo Swee Ann, the I/O pins of the ESP8266 are 5V tolerant. See discussion here: https://hackaday.com/2016/07/28/ask-hackaday-is-the-esp8266-5v-tolerant/ https://hackaday.com/2016/07/28/ask-hackaday-is-the-esp8266-... and official reply on Facebook (of all places): https://www.facebook.com/groups/1499045113679103/permalink/1731855033731442/?hc_location=ufi https://www.facebook.com/groups/1499045113679103/permalink/1... > Teo Swee Ann i can reply officially here: it is 5V tolerant at the IO. while the supply voltage is at 3.3V.