Classical molecular dynamics simulations using simple point charge water potential were performed to obtain high temperature diffusion coefficients for pure water, and ;for O-2, H-2, and OH radical in an infinitely dilute aqueous solution. The simulations were carried out at temperatures ranging from ambient to 973 K and system densities from 0.1 to 1.0 g cm(-3). A logarithmic density expansion of a hard sphere collision model was used to formulate a polynomial fit to the diffusion data and four sets of fitting coefficients are presented. Fourteen coefficients were sufficient to accurately reproduce mass transport coefficients in water at high temperatures and at supercritical conditions. Our parametric fits can be used to estimate diffusion coefficients for these species in a wide range of thermodynamic states, including those relevant to the GEN IV supercritical water cooled nuclear reactor.