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According to the Stokes-Einstein equation the diffusivity of a particle in a gas is a function of the radius of the particle, the absolute temperature, and the viscosity of the gas. Recent theoretical and experimental results are used to extrapolate values for the viscosity of supercritical water vapor up to 800 degrees C. and 2,500 kg. cm. (super -2) pressure. The function Dr s is evaluated in this range, where D is diffusivity and r s the radius of a molecular particle in solution in supercritical water vapor. It is shown that molecular components of rocks should have diffusion rates on the order of 10 (super -3) to 10 (super -4) cm. 2 sec. (super -1) over a wide range of geologic pressures and temperatures. As an example, these results are applied to an evaluation of the diffusivity of silica in supercritical water vapor, and it is shown that the mass transport of silica by diffusion is potentially a relatively rapid geologic process where supercritical water vapor exists at high pressures in the intergranular spaces in rocks, even where the effective directional porosity of the rock is several orders of magnitude less than one. Values for viscosity of water, Dr s , solubility in water of SiO 2 in gm. cm. (super -3) , mass transport of SiO 2 , and mean square displacement of SiO 2 in supercritical water vapor are given graphically for various temperatures up to 800 degrees C. and pressures up to 2,500 kg. cm. (super -2) .