Transport phenomena in biological membranes and in biocatalysts (immobilized enzymes) are influenced by the structure of the matrix. This structure is not invariable but may depend on parameters ofadjacent solutions. Weakly acidic ion-exchange membranes are useful tools for modelling ionic influences on such charged matrices. The pH dependence of tracer permeabilities of [3HJ water and [t'LC'J non-electrolytes was determined on polymethacrylic acid/polyhydroxyethylmethacrylate membranes as a function of their cross-linking and hydrophobicity. At lower buffer concentrations, the non-electrolyte perrneabilities increase between 20 and 500°,0 if the pH is raised from 6.5 to 7.5. The relative ino'ease depends on membrane properties and the molecular weights of the permeants. Water permeabilities were measured between pH 5 and 8, the whole interval of pH controllability of the ion-exchange membranes used. The results are similar to those for the [n4C] non-electrolytes. This control of permeability experimentally established by pH-variation is consistent with assumptions about the influence of the kind of counter ions (H +, Me +, Me 2+) on the degree of dissociation of carboxyl groups within the membrane. Thus the stage of dissociation is responsible for the structure and transport properties of weak ion exchangers.