► We investigate the microchannel with physical and chemical methods together. ► We use the Danckwerts’ model to evaluate the mass transfer behavior. ► Increasing reactant's concentration will increase mass transfer coefficient. ► Volumetric mass transfer coefficient decreases along the channel. ► Volumetric mass transfer coefficient is correlated with slug velocity. An alkaline hydrolysis reaction was used as the model reaction to investigate the performance of liquid–liquid slug flow microchannel. The specific interfacial area was determined through the photographic snapshot method physically by means of measuring the lengths of relevant slugs. The overall volumetric mass transfer coefficients were calculated through the Danckwerts’ model chemically. The influences of various operating conditions on the slug length, the overall volumetric extraction rate and the mass transfer coefficient were investigated quantitatively. A decreasing trend of volumetric mass transfer coefficients along the channel length was found. The linear dependence of the volumetric extraction rate on the volumetric mass transfer coefficient indicates that the overall rate of the process is determined by the mass transfer process. In addition, the volumetric mass transfer coefficients were correlated for different channel lengths.