We performed density functional theory (DFT) calculations and molecular dynamics simulations to determine how water influences the kinetics of H2O2 dissociation (H2O2 = 2OH) in supercritical water (SCW). We assumed that the reaction mechanism in SCW is identical to that in the gas phase. We generated the gasphase potential energy surface for H2O2 dissociation by DFT calculations and thereby determined the reactant geometry and partial charges as functions of the oxygen-oxygen separation distance, which we chose to be the reaction coordinate. From the results of these calculations, we postulated the structure of the transition state (TS) for H2O2 dissociation. We next conducted two sets of molecular dynamics simulations at T-r = 1.15. The first were simulations of dilute solutions of the TS in water, from which we calculated the partial molar volumes for the TS in water. We used these partial molar volumes for the TS and those determined for H2O2 from the preceding study to calculate the activation volume for H2O2 dissociation in SCW, which in turn provided the density dependence of the rate constant. The results show that the rate constant at T-r = 1.15 increases by about 12% as the reduced density of water (rho(r)) increases from 0.25 to 0.75. Between 0.75 < rho(r) < 1.25, the rate constant is insensitive to changes in density. As the water density increases further to rho(r) = 2.75, the rate constant decreases by about 40%. The second set of simulations calculated the change in free energy of solvation along the reaction coordinate for H2O2 dissociation at T-r = 1.15 and rho(r) = 1.25. These simulations revealed that the energy barrier for H2O2 dissociation is 2.1 kJ/mol lower in SCW than in the gas phase. This difference in the activation barrier results in the rate constant at T-r = 1.15 for H2O2 dissociation in SCW being 1.4 times higher than the high-pressure rate constant in the gas phase. The key results from the present study are that the rate constant goes through a maximum with increasing water density and that the rate constant in SCW is larger than the rate constant in the gas phase. Both of these results are consistent with the limited experimental data for this reaction in SCW.