Changes in permeability of granite were measured as water flowed through samples in a temperature gradient. The experimental conditions simulated those around the 'very deep hole concept' nuclear waste repository. Temperature was maintained between 200¿ and 310¿C, in a borehole of a cylindrical sample. Confining pressures of 30 and 60 MPa, with corresponding pore pressures of 10 and 20 MPa, simulated depths of burial of approximately 1.2 and 2.4 km, respectively. A small pore pressure gradient enabled distilled water to flow from the borehole (high temperature) to the outside of the sample (low temperature). Tests were run for intact samples with initial permeabilities of several hundred nanodarcies and for samples containing throughgoing fractures, with initial permeabilities of about 1 millidarcy. In all cases, permeability decreased between 1 and 2 orders of magnitude at a rate that increased with higher temperatures. At 200¿C, permeability dropped by an order of magnitude over a 1-month period, whereas at 310¿C, permeability dropped sharply within a few days to 5% of the initial value. The dissolution of quartz and feldspar and redeposition of these minerals within cracks at lower temperature was found to be the major cause of reduction of permeability. If similar processes occurred near a deep hole nuclear waste repository site in granite, then migration of radionuclides away from the site might be suppressed, even if the rock surrounding the waste contained fractures.