We have performed phenol supercritical water oxidation experiments to elicit water's influence on the reaction kinetics. We report, for the first time, both an acceleration and an inhibition of phenol disappearance by water. More specifically, experiments at 380 and 420 degreesC with similar residence times and phenol and oxygen concentrations showed that as the water concentration increased from 1 to 7 M the phenol conversion decreased. As the water concentration increased further from 7 to 22 M, the phenol conversion reached a minimum and then increased as the water concentration increased. Power law rate equations, which have been used in the past, are not capable of modeling water's effect on phenol supercritical water oxidation kinetics. We provide a new rate equation with a different functional form that possesses this capability. Also, by using a detailed chemical kinetics model, we eliminated the possibility that the observed effects of water are due to its acting as a collision partner, reactant, or product in elementary reactions steps.