The role of water in metamorphism is determined by the independent variables rock pressure, temperature, water pressure, and the amount of water present. An analysis of the possible events that evolve in the regional metamorphism of an argillaceous sediment indicates that, if water flow takes place via pore spaces connected to the surface, the water-vapor pressure may be very low and in the extreme case may approach that at the surface. If the pore spaces are isolated, the water pressure may be equal to or slightly greater than the rock pressure, depending upon the strength and permeability of the rock. The mechanics of a representative metamorphic reaction are outlined under the two extreme conditions of water pressure with different flow rates and reaction rates. Water pressures lower than the rock pressure cause metamorphic reactions to run at lower temperatures. Experiments have been made under water pressures less than the total pressure. Differences in the water pressure from rock to rock may produce reversals in the normal sequence of isograds.