The dilatency during deformation at high pressure and temperature and at various pore pressures has been measured in Carrara marble, Solnhofen limestone, and Gosford sandstone. The method consists of monitoring the volume of pore fluid (argon) that flows into or out of the specimen at constant pore pressure. The confining pressure was 300 MPa, temperatures were up to 873 K, and pore pressure ranged from 30 to 280 MPa. Positive dilatency was observed in some cases in all three rocks in the ductile regime up to the highest temperature, provided the difference between confining pressure and pore pressure was relatively small, thus indicating the occurrence of microcracking even under these conditions. However, the behavior was complicated by the collapse of initial porosity in the limestone and sandstone and by microcracking due to intergranular thermal stress upon heating, especially in the marble. The dilatency observations have important implications for kinetics of fluid-rock interactions under geological conditions. ¿ American Geophysical Union 1989