Physical absorption of CO2 bubbles under slug flow has been studied in a THunction microchannel. Based on the absorption rate of gas bubbles, an online photographic method is proposed to determine the mass transfer coefficients. Validity of this method is verified by evaluating bubble expansion due to pressure drop and by comparing the results with literature. The effect of fluid properties on the mass transfer has also been studied with ethanol solutions, which shows that mass transfer coefficient increases with the increase of ethanol concentration. The amount of gas absorbed during the bubble formation process has been measured to be about 2-10% of the inlet gas phase, and is found to linearly scale with the maximum mass transfer rate. For each fluid system, the initial dissolution rates of bubbles differ very little for short contact distance, whereas, the final amount of dissolution only depends on the residence time. (C) 2014 Elsevier Ltd. All rights reserved.