Ultrasonic microreactors were used to intensify gas-liquid mass-transfer process and study the intensification mechanism. Fierce surface wave oscillation with different modes was excited on the bubble. It was found that for slug bubbles confined in smaller microchannel, surface wave oscillations require more ultrasound energy to excite due to the confinement effect. Cavitation microstreaming with two toroidal vortices was observed near the oscillating bubble by a streak photography experiment. Surface wave oscillation at the gas-liquid interface increases the specific surface area, while cavitation microstreaming accelerates the interface renewal and thus improves the individual mass-transfer coefficient. With these two reasons, the overall mass-transfer coefficient was enhanced by 3-20 times under ultrasonication. As for gas-liquid flow hydrodynamics, ultrasound oscillation disturbs the bubble formation process and changes the initial bubble length and pressure drop. (c) 2015 American Institute of Chemical Engineers AIChE J, 62: 1294-1307, 2016