Diffusion coefficients of vanadium ions in cation exchange membranes are of interest because they allow to calculate the ion exchange across the membrane in an all vanadium redox flow battery which leads to undesired cross contamination and energy losses in the battery system. Diffusion coefficients of V2+, V3+, VO2+ and VO2+ ions in CMS, CMV and CMX cation exchange membranes have been determined by measuring the ion exchange fluxes of these ions with H3O+ ions using a dialysis cell. The experimental data are evaluated on the basis of integrated flux equations which require also ion Exchange sorption equilibria obtained already in previous work. The lowest diffusion coefficients are observed in the CMS membrane for all vanadium ions. This membrane turns out to be the most suitable one for being applied in a vanadium battery since it is expected to prevent most effectively cross contamination of vanadium ions.