Recent experiments showed that the rate of dissociation of H2O2 in supercritical water (SCW) is density dependent and faster than its high-pressure limit rate in the gas phase. These observations suggest that water molecules play a role in this reaction in SCW. We performed density functional theory (DFT) calculations and molecular dynamics simulations to investigate the role of water in H2O2 dissociation. We generated the potential energy surface for H2O2-water and OH-water complexes by DFT calculations to determine the parameters in an analytical intermolecular potential model, which was subsequently employed in the molecular dynamics simulations. These simulations were performed at different water densities. They provided the structural properties (pair correlation functions) of dilute mixtures of H2O2 and OH in SCW, from which we were able to calculate the number of excess solvent molecules and partial molar volumes for each solute. We used the partial molar volumes for H2O2 and OH to calculate the reaction volume for H2O2 = 2OH and thereby determined the density dependence of the equilibrium constant for this reaction. The results show that at the reduced temperature of T-r = 1.15 (695 K) the equilibrium constant for H2O2 dissociation is a function of the water density. The mean value of the equilibrium constant changes by less than 5% between 0.25 < rho(r) < 1, but it decreases by an order of magnitude between 1 < rho(r) < 2.75. Knowing the density dependence of the equilibrium constant for this reaction will allow more accurate mechanism-based models of supercritical water oxidation chemistry to be developed. The computational approach applied herein for H2O2 dissociation is general and can be profitably employed to discern the density dependence of the equilibrium constant of any elementary reaction in SCW. There is currently no experimental approach that will provide this information for reactions involving free radicals.