Permeability was measured during creep deformation of a tight sandstone and a granite at upper crustal conditions. With water at 100¿C as the pore fluid, permeability first decreased then increased during loading to peak stress. Thereafer, permeability decreased as much as 50% over time, even though sample volumetric strain indicated continuous dilatant creep. A silica-rich precipitate was found at the ends of the specimens and etch pits found on quartz grain surfaces. From these observations, we infer that permeability changes during creep are a result of the competing processes of dissolution, precipitation and dilatant microcracking. Depending upon deformation rates, this could have ramifications which are beneficial for waste isolation, but deleterious for geothermal energy extraction schemes.