Reactivation of a preexisting weakness in a rock (such as joints or fault surfaces) is governed by the relationship between the coefficients of sliding friction and internal friction and the orientation of the superimposed stress field. Triaxial compression and sliding friction tests were conducted on sandstones with porosities of 6%, 14%, 20%, 21%, 22%, and 28% at confining pressures of 0.1, 5, 10, 30, 50, 70, and 90 MPa under saturated but drained conditions. Results of these tests indicate that porosity has a minimal effect on the coefficient of sliding friction but that the Mohr failure envelopes are greatly affected by porosity. These data are used to plot limiting theta angles (&thgr;a and &thgr;b) versus confining pressure for each sandstone which outline ''stability'' fields for reactivation of a preexisting surface. Plots for the three lowest porosity sandstones (20%) have limiting angles which are greatly affected by confining pressure. These sandstones exhibit four types of deformational behavior: reactivation of preexisting surface, crosscutting shear failure on a new fracture surface, brittle-ductile transitional behavior, or ductile cataclastic flow. The stability field diagrams exhibit a decrease in the range of limiting angles with increasing confining pressure to a singular value at the brittle-ductile boundary. Tests conducted on 35¿ and 45¿ sawcut cores over the same experimental conditions verify the deformational behavior outlined by the stability field plots.