In this work, the mass transfer characteristics of immiscible fluids in the two kinds of stainless steel T-junction microchannels, the opposing-flow and the cross-flow T-junction, are investigated experimentally. Water-succinic acid-n-butanol is chosen as a typical example of liquid-liquid two-phase mass transfer process. In our experiments, the mixture velocities of the immiscible liquid-liquid two phases are varied in the range from 0.01 to 2.5 m/s for the 0.4 mm microchannel and from 0.005 to 2.0 m/s for the 0.6 mm microchannel, respectively. The Reynolds numbers of the two-phase mixture vary between 19 and 650. The overall volumetric mass transfer coefficients are determined quantitatively in a single microchannel, and their values are in the ranges of 0.067-17.35 s(-1), which are two or three orders of magnitude higher than those of conventional liquid-liquid contactors. In addition, the effects of the inlet configurations, the fluids inlet locations, the height and the length of the mixing channel, the volumetric flux ratio have been investigated. Empirical correlations to predict the volumetric mass transfer coefficients based on the experimental data are developed. (c) 2007 American Institute of Chemical Engineers AIChE J, 53:3042-3053, 2007.