This paper offers potential explanations for the inhibition and acceleration by water of phenol supercritical water oxidation rates. We report new experimental data at 380-465 degreesC that reveals the effect of water density on the oxidation reaction. At 380 and 400 degreesC, increasing the water density increases phenol conversion. At 442 and 465 degreesC, the phenol conversion decreases as water concentration increases from 2 to 9 mol/L. At 420 degreesC, the phenol conversion decreases as water density increases from 2 to 8 mol/L, but conversion increases as water density increases from 8 to 13 mol/L. We present new data and analyses showing that ion-ion reactions are not responsible for the observed effects. These effects are, however, quantitatively consistent with a model based on the existence of two competing rate-determining steps. In one possible scenario, one step has an increase in polarity along the reaction coordinate and the other has a decrease. It is also plausible that diffusion limitations at high temperatures and the increase in the dissociation of phenol into phenolate ions may play a part in water's effect on phenol SCWO kinetics.