A finite element based computational fluid dynamics (CFD) model was developed to study the flow patterns within the slugs and mass transfer with and without superimposed chemical reaction between two consecutive slugs in the liquid-liquid slug flow capillary microreactor. Since the slug flow is a series of alternate slugs of one phase separated by the other, a single element consisting of a slug of each phase was considered. The two slugs in a single domain were distinguished by two kinematic viscosities. The effects of various operating conditions on circulation patterns, mass transfer and reaction are discussed in detail. Finally, the results are compared with experimental data from the literature.