The quantification of the amount of water retained in geothermal reservoir rocks allows a more realistic estimation of reserves for vapor-dominated geothermal reservoirs. If the measured desorption isotherm resembles the production characteristics of a geothermal system, understanding of the adsorption/desorption hysteresis will aid the design of reinjection processes. Adsorption/desorption isotherms of both nitrogen and water vapor on Berea sandstone and a graywacke sample from The Geysers well NEGU-17 were measured, Water vapor adsorption was determined at temperatures of 80, 100, 120 and 130 degrees C. Some measurements were also conducted in the presence of added clay. Hysteresis was observed for both nitrogen and water adsorption. However, the behaviors differ in the two cases, indicating the presence of different mechanisms for nitrogen and water adsorption, The observed hysteresis for nitrogen is likely to be caused by structural heterogeneity. However, for water, the situation is more complicated. Apart from structural heterogeneity, chemical interaction of water with the rock surface and changes in the structure of the rock during adsorption are possible sources of contribution to the observed hysteresis. The surface area of the two rock samples was determined from nitrogen adsorption isotherms. The Frenkel-Halsey-Hill (FHH) equation was found to fit the water adsorption isotherms reasonably well. At low relative pressures. adsorption is the dominant process in water retention in the rock samples studied; at higher pressures, capillary condensation is more important. However, there is no clear distinction between these two phenomena.