The tracer diffusion coefficients of Na + , Cs+ , Sr2 + , and Br− in a homogeneous cation‐exchange membrane have been measured in the homoionic counterion states over the external solution concentration range 0.005–1.0 equiv/liter and in the and heteroionic counterion states over the whole equivalent fraction range at 0.02 and 0.10 equiv/liter total external concentration. Steady‐ and transient‐state permeation and diffusion techniques have been used. The results show that the resin material is essentially uniform apart from a small volume of material of low crosslinking which forms a labyrinthine network interpenetrating the main structure. This labyrinthine network is important mainly in the diffusion of Br− at low external concentrations. In concentrations above about 0.02 equiv/liter the diffusion coefficient DBr of Br− does not vary greatly either with concentration or with counterion type. DBr is greatest in the Cs+ state and least in the Na+ state. These observations and the way in which DBr varies with the equivalent fraction of the counterions in the heteroionic states can be explained by considering the expected frictional interactions between Br− coions and the counterions and by the influence of the local variations in electric potential on the movements of the ions.