Dehydration reactions can induce excess pore-fluid pressures that are high enough to affect the mechanical strength and ductility of rocks. Laboratory experiments have been conducted to investigate this phenomenon in dehydrating rocks with porosities that are initially negligible but increase as the reaction progresses. Polycrystalline gypsum samples were first heated under pressure to induce dehydration, then axially loaded after different amounts of reaction. Within the first 1% of reaction, the ultimate strength of the sample decreased and embrittlement was observed. The ultimate strength recovered within 3% of reaction and eventually exceeded that of pure gypsum. Our results indicate that the initial weakening and embrittlement correspond to a pulse of excess pore pressure in the sample interior. Subsequent strengthening is caused by a decrease in pore pressure and the higher strength of the product phase. ¿ American Geophysical Union 1995