The oxidation of phenylethane-1,2-diol by periodate in water at 1° has been studied in the range pH 1–9. The kinetics are similar to those found previously for ethane-1,2-diol and propane-1,2-diol, and are consistent with the formation of a cyclic periodate ester as intermediate. The phenyl group causes an increase in the equilibrium constant for formation of the ester over most of the pH range, and also increases the rate of decomposition of the ester and its acid strength. These effects are attributed to the inductive and mesomeric properties of the phenyl group.