IN recent years two quite independent theories have been developed to account for the observed electrolyte invasion properties of ion-exchange polymers. Scatchard and Freeman1 suggest that in the application of the Donnan law (1), deviations are best interpreted in terms of the mean activity coefficient of the electrolyte in the exchanger phase, ɣ±. where νc is the number of co-ions in a molecule of electrolyte; νg is the number of counter or gegen ions in a molecule of electrolyte, ν = νc + vg; M is the observed counter-ion concentration; m, the concentration of external electrolyte; m, the concentration of imbibed electrolyte; k, a constant; ɣ ± and γ ± are the mean internal and external activity coefficients of the electrolyte, respectively. In accordance with the theory of specific ion interactions2, an expression has been derived (2), in which a and b are constants, to account for variations in ɣ± for ion-exchangers based on styrene-divinylbenzene copolymers. This relationship has also been applied to polymethacrylic acid ion-exchangers3. It is assumed in this treatment that any deviations resulting from non-uniformity of the exchanger matrix are either negligible or can be eliminated by a small correction to the observed results.