Transport of hydrophobic tetraalkylammonium chlorides (tetrabutylammonium, tetrapentylammonium, tetrahexylammonium and tetraheptylammonium) across a porous polytetrafluoroethylene resin film impregnated by 1-octanol between two aqueous phases is interpreted by considering the steady-state flows of both the tetraalkylammonium ion and the chloride ion within both the aqueous and the membrane phases. The parabolic relation between the hydrophobicity and the permeability in the quantitative structure-activity relationship is attributable to whether the transport of both ions within the aqueous phases or that within the membrane phase is the rate-determining step in the cell system. (C) 2016 Elsevier B.V. All rights reserved.