The self-diffusion coefficients (SDCs) of Na+, Cs+, and Ba2+ have been determined in Nafion-117 membrane having mixed cationic compositions. Membranes with different proportions of Na+-Cs+, Cs+-Ba2+, Na+-Ba2+ and Ag+-Ba2+ cations have been prepared by equilibrating with solutions containing different ratios of these cations. The SDCs of the cations (D-Na, D-Cs, D-Ba) and the ionic compositions of the membrane have been determined using a radiotracer method. For the Na-Cs and Cs-Ba systems, the SDCs of the cations have been found to be independent of the ionic compositions of the membrane. In the case of the Na-Ba system, D-Na does not change with ionic composition, while D-Ba has been found to be strongly dependent on the ionic composition of the membrane and decreases continuously with increasing Na+ content in the membrane. Similar results have also been obtained for D-Ba in the case of the Ag-Ba system. The specific conductivities (kappa(imp)) of the membrane in mixed cationic forms have also been obtained from ac impedance measurement and compared with that (kappa(cal)) calculated from the SDC data. For the Na-Ba system, the increment of kappa(imp) with increase in the Na+ content of the membrane has been found to be parabolic, whereas for the Na-Cs system the increment is linear. The reason behind the different behaviors for different types of ionic systems has been qualitatively explained based on different transport pathways of the cations in the membrane.