The generation and maintenance of excess pore pressures in dehydrating gypsum aggregates were investigated using experiments and microstructural analyses. A triaxial deformation apparatus, was equipped with a pore fluid system connected directly to the dehydrating sample. This system was operated in constant fluid volume mode to monitor pore pressure increase under undrained conditions, and in constant pore pressure mode to monitor fluid expulsion under drained conditions. X ray diffraction and backscatter scanning electron microscopy were used to characterize the spatial relationship among gypsum, the product phase bassanite, and the pores. In addition, we measured the permeability and pore compressibility of the starting material and explored the influence of effective and pore pressures, temperature, and axial load on fluid expulsion. Three stages of fluid expulsion and microstructural evolution during dehydration of an initially low-porosity, low-permeability gypsum aggregate are defined: (1) Initially, fluid released by the reaction is trapped in isolated or discontinuous pore networks and high pore pressures are possible. (2) An interconnected pore network eventually develops and fluid readily escapes. (3) Fluid expulsion slows down drastically as the reaction nears completion. As a result of coupling between dehydration and porosity production, both the cumulative volume of fluid expelled and the expulsion rate increase with increasing temperature, effective pressure, and axial load and with decreasing pore pressure. Our hydrological and microstructural data, combined with previous mechanical data, provide a better understanding of the relationship among changes in fluid volume, porosity, and pore pressure excess, and the deformation behavior of a dehydrating system where drainage evolves with time.¿ 1997 American Geophysical Union