A laminar counterflow mass exchange device is an open duct divided into two subchannels of uniform wall concentration by inserting a permeable barrier, in which only the composition of the flow stream varies along the channels. Efficiency improvement in mass transfer has been investigated analytically by using an eigenfunction expansion in power series. The results of enhancement in mass-transfer efficiency are represented graphically and compared with those in a single-flow device (without a permeable barrier inserted). Analytical results show that suitable adjustment of the permeable-barrier position can effectively enhance the mass transfer efficiency compared with the efficiency in single-flow operations.