Yields for H-2, H-center dot atom, and hydrated electron production in beta/gamma radiolysis of water have been measured from room temperature up to 400 degrees C on a 250 bar isobar, and also as a function of pressure (density) at 380 and 400 degrees C. Radiolysis was carried out using a beam of 2-3 MeV electrons from a van de Graaff accelerator, and detection was by mass spectrometer analysis of gases sparged from the irradiated water. N2O was used as a specific scavenger for hydrated electrons giving N-2 as product. Ethanol-d(6) was used to scavenge H-center dot atoms, giving HD as a stable product. It is found that the hydrated electron yield decreases and the H-center dot atom yield increases dramatically at lower densities in supercritical water, and the overall escape yield increases. The yield of molecular H-2 increases with temperature and does not tend toward zero at low density, indicating that it is formed promptly rather than in spur recombination. A minimum in both the radical and H-2 yields is observed around 0.4 kg/dm(3) density in supercritical water.