Binary mass diffusion coefficients and thermal diffusivities for hydrothermal sodium nitrate solutions as a function of pressure (270 < P < 1000 bar), temperature (400 < T < 500 degrees C), and concentration (0.25 < C < 3.0 m) were measured by the laser-induced grating technique, In concentrated hydrothermal NaNO3 systems, the critical slowing down was significant as far as 300 bar from the phase-separation pressure, resulting in binary diffusion coefficients near the critical point that are comparable to values at ambient conditions, Further from the critical point the mass diffusion coefficients plateaued at their ordinary values. Ordinary binary mass diffusion was about 15 times faster than at 25 degrees C and atmospheric pressure. The ordinary binary mass diffusion coefficients were compared with predictions from hydrodynamic diffusion theory. Experimental results agreed well with predictions from the Stokes-Einstein equation, where the diffusing species was best represented by a hydrated contact ion pair. The Wilke-Chang correlation also yielded good predictions when the solute molar volume was defined as the volume of the hydrated contact ion pair. Predictions can be improved by about 10% if the degree of association can be calculated. Thermal diffusion coefficients (Soret effect) at 450 degrees C were also measured and are about 250 times faster than at ambient conditions. The laser-induced grating technique was found to be highly complementary to the Taylor dispersion technique for diffusion measurements in hydrothermal systems. The grating technique;works best at higher concentrations and near critical points, the two regimes where the Taylor dispersion technique becomes increasingly more difficult.