Hydrodynamics and mass transfer in liquid-liquid slug flow inside a square microchannel (MC-1, aspect ratio 1) and a rectangular microchannel (MC-2, aspect ratio 2) with identical hydraulic diameter (400 pm) were investigated experimentally. The results showed that the overall volumetric mass transfer coefficient (K(L)a) in MC-2 was 16.2% lower than that in MC-1 due to enlargement of the dispersed phase slugs in MC-2, leading to development of stagnant zones within plugs and reduction of wall film renewal. Three different junction configurations (JC) were studied. K(L)a was enhanced by up to 34.8% using JC-1, compared to JC-2 or JC-3 due to greater movement of the biphasic interface, resulting in higher mixing within slugs using JC-1. The junction arrangement influenced not only the mass transfer during the slug formation stage, but also the slug-flowing stage due to effects on the vortex patterns along the microchannel. (C) 2017 Elsevier B.V. All rights reserved.