Equilibrium NaCl uptake studies in crosslinked and non-crosslinked cation-exchange membranes composed of sulfonated poly[bis(3-methylphenoxy)phosphazene] have been performed at 25 degrees C for external salt solutions ranging in concentration from 0.1 to 1.0 M. Membrane swelling, membrane fixed-ion concentration, and membrane colon (anion) concentration were determined for polyphosphazene membranes with ion-exchange capacities of 0.60, 1.22, and 1.40 mmol/g and for a Nafion 117 perfluorosulfonic acid membrane. For the polyphosphazene membranes, colon intrusion decreased with increasing membrane fixed-ion concentration and with polymer crosslinking, but was higher than that with Nafion. Small angle X-ray diffraction was used to model the micro-morphology of the polyphosphazene membranes in terms of a cluster-network structure. Neither the dry polymer ion-exchange capacity nor the wet membrane fixed-ion concentration were accurate measures of the colon exclusion capability of the polyphosphazene membranes, relative to Nafion. It was found, however, that colon exclusion in crosslinked and non-crosslinked polyphosphazene membranes and in Nafion 117 correlated linearly with the fixed-charge/volume ratio of the ionic clusters.