The rates of several radiation-induced reactions are assessed via pulse radiolysis in order to extend a model for nuclear reactor coolant radiolysis to supercritical conditions. We find changes in radiolysis yields and significant deviations from Arrhenius behavior at 250 bar as the temperature approaches and exceeds the critical temperature of pure water. At 380°C we also observe a strong pressure dependence of the reaction rates of ions and hydrophobic species. Using a homogeneous chemistry model, we find by 350°C that the relatively mild changes in these reaction rates increase the predicted critical hydrogen concentration relative to 325°C.