Data on the component composition and acid-base properties of cooling fluid in the systems H 2 O-CO 2 , H 2 O-NaCl, and H 2 O-rock forming minerals, which were derived by minimizing the Gibbs free energy, and the evaluated contributions of major fluid components to its acid-base properties suggest that the main reason for the inversion of acid-base properties of magmatic fluids in the fluid-granite system is the opposite tendencies in the changes of the dissociation constants of acidic and basic fluid components in the course of fluid cooling.