Application of thermodynamics to the study of geologic systems permits prediction of equilibrium relations among minerals and aqueous species at pressures and temperatures to 5 kb and — 1000°C. The predictions afford a frame of reference for interpreting phase relations in geologic systems and assessing thermodynamic constraints on reaction rates and interphase mass transfer in geochemical processes. For example, consideration of equilibrium constraints for coexisting minerals, aqueous fluids, and H20-saturated igneous melts permits estimation of the composition of hydrothermal fluids at magmatic temperatures and pressures. Similarly, transition state theory and computer experiments can be used to assess the rates at which silicates react incongruently with aqueous solutions in geochemical processes. Calculations of this kind indicate that these rates are controlled primarily by the chemical affinities of the hydrolysis reactions. As a consequence, the rates of all such irreversible reactions for an assemblage of rock-forming minerals are coupled, and the rate at which each mineral in the reactant assemblage equilibrates is controlled by the last mineral to reach equilibrium. Calculation of interphase mass transfer can be carried out as a function of time by taking account of constraints imposed by conservation of mass and partial and local equilibrium, which permits numerical integration of differential equations representing changes in composition as a function of reaction progress. The results of such calculations can be compared with phase relations observed in rocks to refine the predictions and interpret better the geologic record.