An accurate predictive model for the long-term strength of the continental lithosphere is important in a range of geophysical and geodynamic problems. While laboratory experiments are consistent with Mohr-Coulomb brittle faulting in the cold, upper continental crust, there is increasing evidence that time-dependent processes any also be important in these rocks, even at low temperature. However, there is some ambiguity as to the exact form of the constitutive law for describing time-dependent behavior of upper crustal rocks. Here we present results of room temperature creep experiments on a suite of water-saturated sandstones spanning a range of petrophysical and rheological properties and underlying deformation mechanisms. On physical and microstructural grounds our analysis suggests that a modified power law creep, of the form ϵ˙=A'(&sgr;d-&sgr;f)&eegr;', where &sgr;d is the differential stress and &sgr;f is the long-term failure (fundamental) strength, provides a more complete description of the experimental data. In particular, the parameters can be used to differentiate between sandstone types, with A', &sgr;f, and &eegr;' varying systematically with cementation state, rock rheology, and confining pressure. The fundamental strength (&sgr;f) for time-dependent deformation varies much more than the other parameters of the distribution, making it a potentially sensitive indicator of underlying creep mechanisms. Further tests would be needed to prove the constitutive law on a wider range of rock types and to prove that the three-parameter model is statistically better in the general case. ¿ 2001 American Geophysical Union