A solute was transferred from an aqueous solution to deionized water in a microchannel device with a rectangular channel of 200 lm × 200 lm. The two liquid layers flowed in parallel within the rectangular channel. After making contact in the channel, the layers were split into two streams through a knife-edge. The amount of solute transferred was determined by analyzing liquid samples taken from the outlet. Thus, conventional analytical instruments were readily available for obtaining the total concentration. Mass tansfer characteristics were examined through the measurement of diffusion coefficients for several solutes. Although the two liquids were carefully supplied at the same velocity, equal flow splitting was difficult to obtain. The unequal splitting led to scattering of the observed solute concentrations at a fixed supply flow rate. This could be a serious drawback as a mass tansfer device; however, a method was proposed to choose an appropriate value for equal splitting. The ratio of effluent flow rate of one liquid to another was changed intentionally. The solute concentration in the receiving liquid for the flow rate ratio of unity corresponds to the data for equal flow splitting. On the basis of the solute concentrations defined, the diffusion of benzoic acid was successfully analyzed using a conventional penetration model with a correction parameter. Furthermore, diffusion coefficients for sucrose and glycine were determined using the basic equation obtained with benzoic acid. The observed values compared fairly well with reported ones. The correction parameter expresses characteristics of the microchannel device; however, the physical picture is still unclear.