The photoreduction of hydrogen peroxide in water under hydrogen of up to 100 atm pressure has been investigated. The reaction involves a chain mechanism with the quantum efficiency for the disappearance of hydrogen peroxide being strongly dependent upon the concentration of hydrogen peroxide. A maximum quantum efficiency of about 35 mol/einstein occurs at a hydrogen peroxide concentration of about 5 x 10/sup -4/ M. At higher hydrogen peroxide concentrations the reaction (HO. + H/sub 2/O/sub 2/ ..-->.. H/sub 2/O + HO/sub 2/.), which leads to chain termination by subsequent HO/sub 2/. reactions, is competitive with the chain-propagating reaction (HO. + H/sub 2/ ..-->.. H/sub 2/O + H.) so that the quantum efficiency decreases with increasing (H/sub 2/O/sub 2/). At lower hydrogen peroxide concentrations the chain-propagating step (H. + H/sub 2/O/sub 2/ ..-->.. H/sub 2/O + HO.) cannot compete with chain termination by (H. + O/sub 2/ ..-->.. HO/sub 2/.) so the yield again decreases. Scavenging of hydrogen atoms at lower hydrogen peroxide concentrations is not well understood. The participation of HO/sub 2/. radical is inferred from the observation of a strong pH effect which shows an inflection point at a pH near the pK/sub a/ of HO/sub 2/. (4.88). Amore » complex mechanism for the reaction is proposed and analyzed by numerical integration of the resulting rate expressions. This part of the work is an example of the usefulness of recently developed numerical integration techniques in the analysis of complex nonlinear reaction mechanisms.« less