Diffusion coefficients of water as a solvent and ethanol as a tracer in sulphonated polyethylene (SPE) were measured by the pulsed-gradient spin-echo NMR method. Comparison with the data available for a number of ionic and non-ionic polymers and solvents shows that, except for Nafion, all the materials exhibit a fairly universal dependence of diffusivity on the volume fraction of the solvent. This result is explained on the basis of the geometric obstruction approach and general conditions are formulated for the observed universal dependence. Explanations are proposed for the peculiar behaviour of Nafion and for the low diffusivity of ethanol within the same framework.