The results of an experimental study on the gas-liquid flow structure and interphase mass transfer are presented for a periodic slug flow of ethanol-CO 2  mixture in a straight microchannel and a microchannel with a sudden change in the cross-section. The experiments were carried out for a microchannel with successive contractions and expansions along its length and average cross-section of    m 2 . For determination of the change in the volume of the gas slug along the microchannel length because of CO 2  absorption, the method of high-speed flow visualization was used, followed by digital image processing. The frequency of generation, speed, size, and volume variation of the gas bubbles were measured along the length of the channel. From the data obtained, the volumetric mass transfer coefficient from the liquid side was determined. A comparison was made of the obtained volumetric mass transfer coefficient for the straight channel and for the channel with sudden enlargement of the cross-section: slight enhancement of the mass transfer in the channel with the sudden change in the cross-section, resulting from the pulsating shape of the bubbles, has been shown.