The understanding of the hydromechanical behaviour of crystalline rocks with temperature and pressure is necessary for assessing the long-term performance of deep underground storage facilities. Permeability and porosity variations with pressure and temperature were investigated on predominantly granitic samples in the laboratory. The test results were also used to estimate the thermal fracturing characteristics of the samples. Permeability variation with pressure was influenced significantly by rock type, physical condition of test samples, and stress paths followed during testing; it was however, independent of temperature path. Permeability of the granitic samples decreased with increasing temperature between 25°C and 100°C, then increased rapidly to the maximum test temperature. The residual effect of temperature on the granitic samples resulted in increased porosisty and permeability. The threshold temperature of significant thermal crack initiation ranged between 80°C and 125°C for the granite. Thermal cycling, and increased presssure increased and reduced this temperature range, respectively.