A permeable barrier was inserted to divide an open duct into two subchannels of uniform wall fluxes for conducting double-pass laminar countercurrent operations. The mass transfer rate of such a double-pass device was substantially improved and has been investigated theoretically by using an eigenfunction expansion in power series. The theoretical predictions of mass-transfer efficiency enhancement in double-pass parallel-plate mass exchangers were represented graphically and compared with those in an open duct of single-pass operation (without a permeable barrier inserted). The results show that the double-pass operation can effectively enhance the mass transfer efficiency and especially when the permeable-barrier position is appropriately adjusted.