Donnan concentration, concentration in the pore liquid, self-diffusion coefficient and mobility of the sulphate co-ion in sulphonated polystyrene grafted onto a polyethylene membrane as backbone have been determined using 0.001-0.50 M MgSO4 as external electrolyte. In the dilute solution region the Donnan concentrations of the co-ion are found to be less than the corresponding, experimentally obtained concentrations in the pore liquid. In the dilute solution region higher Donnan exclusion is also observed when calculated in terms of concentrations rather than activ ities. The values of the above-mentioned parameters for the sulphate co-ion in the present membrane are found to be smaller than those for halide and sulphate co-ions in so-called homogeneous membranes. Higher co-ion charge, and the heterogeneous nature and low water content of the present membrane, are thought to be the reasons for the observed smaller values. The values of the above-mentioned parameters increase with increasing temperature at a particular solution concentration. © 1987.