Flow pattern, pressure drop, and mass transfer characteristics have been studied for the gas-liquid two-phase flow in a 1.0 mm inner diameter circular microchannel reactor. A mixture Of CO2, N-2, and polyethylene glycol dimethyl ether was used to represent the gas and liquid phases, respectively. Bubbly, slug, churn, and slug-annular flow patterns were observed in the present work. A flow pattern map using superficial gas and liquid velocities as coordinates has been developed and compared to the existing flow pattern maps for similar to 1 mm diameter channels. The data obtained for the pressure drop of the two-phase flow were analyzed and compared with the homogeneous model and the separate flow model to assess their predictive capabilities. The liquid side volumetric mass transfer coefficient increased with an increase of the superficial gas and liquid velocities, and the influences of the superficial gas and liquid velocities on it were demonstrated. The liquid side mass transfer coefficient, which was as high as 3.34 s(-1), was 1 or 2 orders of magnitude higher than the traditional industrial gas-liquid contactors.