The stopped flow spectrophotometric method was applied to study the kinetics of the ozonation of o-cresol in aqueous solutions of pH values varying from 2 to 9 at 10 to 40 degrees C. The fast reaction between o-cresol and ozone is second order overall with first order in each reactant. The ozonation rate increases with the temperature and pH value of the solution. In neutral solutions, the overall rate constant increases from 422,000 M(-1)s(-1) at 10 degrees C to 1,549,000 M(-1)s(-1) at 40 degrees C. The activation energy is about 30 KJ/mol in the neutral solutions; it increases slightly with the acidity of the solution.A mechanism based on the initial attack of ozone molecules at the ortho and para positions of o-cresol is proposed for the ozonation reaction. According to this mechanism, three moles of ozone are required to react with each mole of o-cresol resulting in the rupture of the aromatic ring and production of various acids. The oxidation products were identified by these and other investigators. The proposed mechanism yields a second order kinetics for the overall reaction, as confirmed by the kinetic experiments.