The effects of pH and dissolved O 2 on the γ-radiolysis of water were studied at an absorbed dose rate of 2.5 Gy s −1. Argon- or air-saturated water with no headspace was irradiated and the aqueous samples were analyzed for molecular radiolysis products (H 2 and H 2O 2) as a function of irradiation time. The experimental results were compared with computer simulation results using a comprehensive water-radiolysis kinetic model, consisting of the primary radiolysis production, subsequent reactions and related acid–base equilibria. Both the experimental and computer model results were discussed based on the steady-state kinetic analysis of smaller reaction sets consisting of key production and removal reactions. While the main production path for a water decomposition product is the primary radiolysis, the main removal path varies. For H 2O 2 the main removal path is the reactions with e aq − and OH, whereas for H 2 it is the reaction with OH. As a result, the presence of a dissolved species, or a change in chemical environment, affects the concentrations of H 2O 2 and H 2 through interaction with radicals e aq − and OH. Over a wide range of conditions, there exist quantitative but simple relationships between the radical and the molecular product concentrations. The experimental and model analyses show that dissolved oxygen increases the steady-state concentrations of H 2O 2 and H 2 by reacting with OH and e aq –, and the impact of oxygen is more noticeable at pH below 8. The steady-state concentrations of water decomposition products are nearly independent of pH in the range 5–8. However, raising pH above the pKa value of the acid–base equilibrium of H (⇆ e aq −+H +) significantly increases [H 2O 2] and [H 2] at the expenses of [ OH] and [ e aq –]. At pH >10, the radiolytical production of O 2 becomes significant, but at a finite rate. This considerably increases the time for the irradiated system to reach a steady state, and is responsible for different impacts on [H 2O 2] and [H 2] due to radically produced O 2, compared to impacts due to initially dissolved O 2. Model sensitivity analysis has shown that at higher pHs (pH >10) transient species such as O 2 − and O 3 − play a major role in determining the steady-state concentration of molecular products H 2 and H 2O 2. Further validation of the water radiolysis model, particularly at higher pHs, is also discussed.