The crystallization of some hydrous magmas can be modeled, in part, via the reaction between hydrous melt and anhydrous minerals to yield hydrous minerals. The reactions (herein termed "hydration crystallization") are equilibrium, incongruent, vapor-absent crystallization reactions, the reverse of dehydration-melting reactions. The principal petrographic evidence for hydration crystallization is partially reacted and resorbed pyroxene and oxide minerals mantled by amphibole and biotite. Hydration crystallization can buffer the water content of mildly hydrous (up to ∼2 wt% H2O) magmas at values below saturation, conceivably to the completion of crystallization, thus contradicting the maxim that "all magmas go to water saturation." Even if the buffering effect does not persist throughout the crystallization history, water contents of magmas in which these incongruent reactions occur will remain lower than magmas in which they have not. Whether they preclude vapor saturation, equilibrium hydration crystallization reactions control the late magmatic evolution of many hydrous plutons.