Supercritical water oxidation (SCWO) experiments were performed in a tubular flow reactor at 420–465°C and 141–241 bar. Phenol was the organic reactant. Both pure water and a helium (1/3 by mol)–water mixture served as reaction media. Adding helium to the reaction medium permitted variation of the water concentration and system pressure independently. By decoupling the water concentration from the system pressure, it was shown that the rate of phenol disappearance during SCWO is influenced by the water concentration and not the system pressure. The experiments consistently revealed that adding helium, and thereby decreasing the water concentration at a fixed system pressure, increased the phenol conversion. In addition, experiments showed that lowering the water concentration using pure water as the solvent also increased the phenol conversion. The results suggest that dilution with helium (and perhaps other inert gases) may be a new way to control SCWO reaction rates.