Mass transfer of an acetic acid-methanol system in a microchannel was measured by laser Raman spectrometry. The Raman peak intensities of the molecules were proportional to the concentrations for each species, and hence the concentrations could be quantitatively determined from the Raman spectra. By measuring Raman spectra at specific spots in the microchannel placed on the X-Y-Z moving stage, we obtained two-dimensional concentration profiles in a microreactor made of a Y-shaped channel. The observed concentration profiles of acetic acid and methanol in the microreactor with a large aspect ratio (defined as width/depth) were coincident with the calculated ones at several flow rates. We confirmed the reliability of both our measurement technique and CFD analysis method. The CFD calculation gave the concentration profiles in microreactors with different aspect ratios. The CFD analysis revealed that the deformation of the fluid interface strongly depends on the aspect ratio of the microreator. Deformation of the fluid interface was small for the aspect ratio >3 and <0.3, and large for the aspect ratio congruent to 1. We found that Y-shaped channels with aspect ratios larger or smaller than unity are suitable for measurement of mass transfer in microreactors by laser Raman spectrometry.