In this thesis, we experimentally studied the mass transfer during CO<sub>2</sub> absorption into water, ethanol, methanol and silicone oil under slug flow in microchannels. We showed that the initial bubble size is determined by the liquid fraction and channel geometry, while the CO<sub>2</sub> diffusion rate is determined by the gas pressure and liquid properties, such as the Henry's constant and the diffusion coefficient. The reduction of the gas void fractionΑ<sub>G</sub> along the flow direction and the transformation of segmented flows into dilute bubbly flows was observed and predicted. In high viscosity liquids, we showed the liquid film thickness is related to the capillary number and the gas pressure. We also constructed experimental setup for investigating CO<sub>2</sub> cavitation in microchannels. A linear time dependence of bubble growth from depressurization is observed. In addition, we proposed the fabrication procedure of co-flowing capillary tip and listed its current limitations.