Abstract The creep strength of porous and fully dense solids can differ in substantial ways, and thus it is of geologic interest to investigate the effect of porosity on the deformation of earth materials. In our experiments, porous monomineralic aggregates of both quartz and calcite were isostatically hot-pressed for various lengths of time under pressures of up to 600 MPa and temperatures of up to 800°C for calcite and 1250°C for quartz. Densities achieved were 0.993 theoretical for calcite and 0.89 for quartz. The apparent activation energy for hotpressing was 40 kcal/mole ± 20 for quartz and 5 kcal/mole for calcite. The mechanisms of densification for quartz under these experimental conditions appeared to contain a substantial diffusional component, although this was not proven conclusively. The addition of water as a pore fluid substantially increased the densification rate. Some of the calcite samples showed rapid decreases in the final densities reached at higher temperatures, which were interpreted as having been caused by decarbonation as a result of the lithostatic load and the low CO 2 pressure in the pore space. This decarbonation occurred at substantially lower temperatures than would be expected if the sample were fully dense. Both these experiments and previously published discussions of hot-pressing seem to indicate that the densification rate will be strongly affected by the porosity present, and that it will be very difficult to predict the creep deformation properties of porous earth materials using theory or experiments on fully dense minerals alone.