Isothermal interdiffusion coefficients have been obtained by the Gouy interferometric free-diffusion method for the system NaCl-MgCl2-H2O at 35 degrees C. The solute molality ratios were 3:1, 1:1, and 1:3 at total molar solute concentrations of 3.78, 3.70, and 3.58 mol dm(-3), respectively. These total molarities correspond to three nearly the same molalities for which data are available at 25 degrees C. The results suggest that within experimental error the four diffusion coefficients of this ternary system at 35 degrees C are about 1.25 times larger than their corresponding values at 25 degrees C, which is close to the fluidity ratio for pure water at these same two temperatures. However, the ratio for the main term diffusion coefficients D-11 and D-22 appeared on the average to be a little greater than this value (1.28 and 1.26), and the ratios for the cross-term diffusion coefficients D-12 and D-21 appeared to be a little less than this value (1.21 and 1.24).