The present study investigated the influence of square microchannel (MC) size on hydrodynamics and mass transfer in the liquid-liquid slug flow regime. Three square MCs with the hydraulic diameters of 200, 400, and 600 m were used. The employed method for estimating mass-transfer coefficients remarkably increased the accuracy of the results. The findings revealed that decreasing the MC size improved the interfacial area due to plug length enlargement and deteriorated mass-transfer resistances because of augmented internal circulations, leading to the considerable enhancement of mass-transfer coefficients. The increasing effect on the overall mass-transfer coefficient became greater with flow velocity, showing that size effect on mass-transfer resistances was more profound at higher flow velocities. The influence of size on the interfacial area was significantly greater than that on mass-transfer resistances due to the significant increment of wall film length with the decrease in channel size. (c) 2017 American Institute of Chemical Engineers