A method for humidifying a fuel stream for a direct methanol fuel cell. An ultrasonic transducer (122) is used to create a vapor of methanol without heating. Water is also vaporized with an ultrasonic transducer (132) to create a vapor of water without heating. The water vapor and the methanol vapor are combined (240) in a certain ratio to form a humidified fuel vapor which is presented to an anode (112) of the direct methanol fuel cell (110). The ratio of water to methanol can be adjusted automatically by a sensor (180) that monitors the current drawn by the load (170) on the fuel cell.