This report presents a systematic investigation of the influence of fixed charge group concentration on salt diffusion coefficients in ion exchange membranes. Cross-linked cation and anion exchange membranes (CEMs and AEMs) having different fixed charge group concentrations and similar water content were synthesized via a one-step free radical copolymerization reaction. Concentration gradient-driven ion transport through the membranes was probed by measuring salt permeability coefficients as a function of salt concentration in the upstream solution. For all membranes, salt permeability coefficients increased by approximately one order of magnitude as external solution salt concentration increased from 0.01 to 1 M, predominantly due to similar increases in salt partition coefficients. On average, salt permeability coefficients for both series of membranes decreased with increasing fixed charge group concentration to nearly the same extent. Apparent salt diffusion coefficients, which were extracted from salt permeability and salt partition coefficients via the solution-diffusion model, changed to a greater extent for the AEMs than those for the CEMs despite similar changes in membrane fixed charge group concentration. The relative changes in apparent salt diffusion coefficients between AEMs and CEMs were attributed to differences in free volume of the membranes. This hypothesis was supported by positron annihilation lifetime spectroscopy measurements, which demonstrated variation in ortho-positronium lifetime values (a measure of free volume element size) for these membranes that correlated with variations in apparent salt diffusion coefficients.