The radiation chemical yields of hydrogen peroxide formed in the gamma-radiolysis of water with scavengers for oxidizing and/or reducing radicals were measured to examine initial water decomposition pathways to oxidizing species. Hydrogen peroxide yields were found to decrease toward zero with increasing concentration of OH radical scavenger in all solutions, suggesting that the OH radical is the sole precursor to hydrogen peroxide. The yields of hydrogen peroxide in nitrate and selenate solutions are closely associated with the scavenging capacity of the precursor to the hydrated electron, suggesting that its reactions have a significant role in hydrogen peroxide formation. Observed hydrogen peroxide yields at high nitrate concentrations coupled with model calculations show that the molecular cation of water, H2O+, is the dominant precursor of the oxidizing species leading to hydrogen peroxide. Proton-transfer reactions of the water molecular cation give 79% of the oxidizing species, whereas other reactions such as dissociative recombination reactions account for the rest. There is a significant additional production of OH radicals in the radiolysis of selenate solutions due to the production of O-.(-) and the scavenging of low-level excited water molecules.