Quantitative kinetics for the alkylation reactions of phenol and p-cresol with tert-butyl alcohol in near-critical water as a function of temperature are reported. The reaction products of the alkylation reactions were ortho- and para-substituted phenols, which are used extensively as antioxidants in numerous polymer systems. A new experimental reaction apparatus was designed, constructed, and successfully operated to allow pressure control and sampling of the reaction systems, which provided rate constants in the temperature range 250–300°C. Additionally, activation energies and heats of formation were calculated from the temperature dependence of the rate constants, and ortho-substitution was found to be exothermic whereas para-substitution was found to be endothermic. These results demonstrate that product distribution of the alkylation reactions could be tuned with temperature to produce specific products.