The formation of H-2 in the radiolysis of liquid water is more closely associated with the nonhydrated electron or precursor to the hydrated electron than with the hydrated electron. Experiments with 5 MeV helium ions suggest that the principle mechanism for hydrogen formation is dissociative recombination of the water cation and the nonhydrated electron. The nonhydrated electron undergoing recombination does not appear to be in the lowest energy p-like state, but it probably has an excess energy of several eV, The second-order recombination processes is very sensitive to the type of ionizing radiation leading to the observed variation in hydrogen yields. Furthermore, the relative mobility of the reactants through heterogeneous interfaces may result in a substantial yield of molecular hydrogen in practical situations involving adsorbed water.