Isothermal mutual diffusion coefficients (interdiffusion coefficients) were measured for ternary aqueous mixtures of NaCl and Na2SO4 at a constant total molarity of 1.000 mol . dm(-3) and 298.15 K. Measurements were performed using Rayleigh interferometry with computerized data acquisition at NaCl molarity fractions of z(1) = 1, 0.90, 0.75, 0.50, 0.25, and 0. Densities of the solutions were measured with a vibrating tube densimeter. At all ternary solution compositions, one cross-term diffusion coefficient has negative values whereas the other has positive values. These measurements supplement our earlier results at 0.500 mol . dm(-3). Both main-term diffusion coefficients are significantly smaller at 1.000 mol . dm(-3) than at 6.500 mol . dm(-3) at any fixed value of z(1), whereas both cross-term coefficients are shifted in a positive direction. Trace diffusion coefficients D*(Cl-) and D*(SO42-) were extrapolated from these results for the Cl-(aq) ion in 1.0 mol . dm(-3) Na2SO4(aq) and for the SO42-(aq) ion in 1.0 mol . dm(-3) NaCl(aq). Values of D*(Cl-) in Na2SO4(aq) and in NaCl(aq) were found to be essentially identical, as were D*(SO42-) in these same two electrolytes, provided the comparisons were made at the same volumetric ionic strengths.