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 0.500 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*(S ) were extrapolated from these results for the Cl-(aq) ion in 1.0 mol·dm-3 Na2SO4(aq) and for the S (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*(S ) in these same two electrolytes, provided the comparisons were made at the same volumetric ionic strengths.