Dehydration and hydration reactions can significantly change permeability and fluid pressure in fault zones. Conversely, sudden drops in fluid pressure caused by faulting, or gradual increases in fluid pressure caused by fault sealing, can cause subsequent long-term dehydration or hydration of minerals in fault zones. Reactions with gentle Clapeyron slopes are most susceptible to changes in fluid pressure and produce nonintuitive results-dehydration reactions reduce fluid pressure, whereas hydration reactions increase fluid pressure. Dehydration and hydration reactions also influence fault-valve behavior. The strength of the effect depends on the rate of reaction relative to the rate of fault sealing. Reactions with steep PT slopes operate together with sealing processes to decrease PH2O, but they oppose the decrease in porosity caused by sealing. Reactions with gentle PT slopes work in concert with sealing to change PH2O and porosity. Regardless of the type of reaction, where reaction rates exceed sealing rates, PH2O can be maintained near the equilibrium pressure of the reaction until the reactant minerals or fluids are exhausted, sharply modifying the timescale for earthquake recurrence.¿ 1997 American Geophysical Union