A heat-transfer experiment has been conducted in marine pelagic clay to provide data to model geomechanical and geochemical properties of seabed sediment exposed to a relatively large thermal gradient and sea-floor pressure. This experiment entailed emplacement of a resistance-type heat source in the center of a tank of sediment (1 m3) from the central North Pacific and overlain by a reservoir (2 m3) of artificial seawater. The system was pressurized to 500 bar. Pore-fluid samples extracted from sediment at 5-150°C and samples of sediment altered at temperatures from ∼ 30° to 270°C yield information on mass transport and sediment alteration processes. Sediment alteration in the vicinity of the heat source caused an increase in sediment shear strength, water and Na2O contents, and formation of hydrated analcime at the expense of kaolinite and quartz. This so-called "baked" zone consisted of sediments altered at temperatures of > 170°C. The high Na2O contents of baked-zone sediments require additions of Na+ in amounts exceeding that that available in pore fluid alone and suggest convective circulation of sea-salt solution along the relatively permeable boundary between the sediment and the heat probe. Quartz dissolution and removal of SiO2 by convectively circulating sea-salt solution, moreover, played an important role in microcrack extension and permeability enhancement in the baked zone, itself. Mineralogic and chemical changes in baked-zone sediment provide useful insight into the mechanism of alteration of hemipelagic sediments in contact with dolerite sills in the Guaymas Basin, Gulf of California. Sediment exposed to temperatures of < 170°C remained unconsolidated even though important mineralogic and chemical changes occurred. Sediments from the highest temperature portion of this zone contained chlorite and calcite(?) and had lost Na- and K-rich phases. In situ sampled pore fluids were characterized by distinct compositional gradients including a large decrease in Na+, Cl-, Mg2+, Ca2+ and SO2-4 concentrations with increasing temperature. These changes are consistent with thermodiffusional transport (so-called Soret effect) and rock-water interaction. Cl- was the only component to respond only to thermal diffusion and correspondingly was unaffected by rock-water interaction. Application of results from the heat-transfer experiment to the subseabed disposal concept shows that thermal diffusion will dominate near-field mass transport, provided sediments remain relatively unconsolidated and impermeable and are not affected by convection. Assuming convection does not occur, or if it does, is localized and closed to recharge pathways, pH at temperatures of > 250°C will most probably be mildly acidic. The redox intensity (fO2) will be constrained by sediment chemistry and mineralogy.