Isothermal mutual diffusion coefficients (interdiffusion coefficients) and refractive index increments were measured for ternary aqueous mixtures of NaCl and Na2SO4 at constant NaCl molarity fraction z(1) = 0,9500 at 298.15 K, using high precision Rayleigh interferometry with computerized data acquisition. The densities of these solutions were measured using vibrating tube densimetry. The experiments were performed at seven total molarities ranging from (0.499 92 to 4.978 34) mol(.)dm(-3). These measurements supplement our earlier ones at other compositions and composition fractions. The Na2SO4 cross-term diffusion coefficient D-21 is small and negative at nearly all of the ternary solution compositions, whereas the NaCl cross-term diffusion coefficient D-12 is larger and positive. However, at z(1) = 0.9500, D-21 changes sign as the total concentration exceeds 4.3 mol(.)dm(-3), and above 3.2 mol(.)dm(-3) the value of D-12 exceeds that of the corresponding Na2SO4 main-term diffusion coefficient D-22. Thus, at high concentrations, a concentration gradient of Na2SO4 causes the transport of more NaCl than of itself. At each fixed total concentration, extrapolation of Na2SO4 main-term diffusion coefficients from these and our earlier results, to zero concentration fraction of Na2SO4 (i.e., z(1) --> 1), yielded the trace diffusion coefficient of the SO42- ion D-tr(SO42-) in NaCl(aq). Comparing these D-tr(SO42-) values in NaCl(aq) to published D-tr(SO42-) values in Na2SO4(aq) shows agreement to 1-8%, when the comparisons are made at equal volumetric ionic strength. Similar extrapolations of the volumetric and refractive index increment results yielded the trace partial molar volume and trace refractive index increment, respectively. Densities of NaCl(aq) at 298.15 K were critically reanalyzed.