A kinetic study of the degradation by ozone of eight phenolic acids present in wastewaters from olive oil mills has been performed by using a competition kinetic method. The selected phenolic acids are : caffeic, p-coumaric, syringic, vanillic, 3,4,5-trimethsxybenzoic, veratric, p-hydroxybenzoic, and protocatechuic. The influence of the operating variables (temperature, pH, and ozone partial pressure in the gas stream) is established, and the stoichiometric ratios for the individual direct reactions between ozone and each acid are determined. Once the reaction rate constants are evaluated, they are correlated as a function of temperature and pH into kinetic expressions which are provided for every phenolic acid. The global process occurs in the fast and pseudo-first-order kinetic regime of absorption, a condition required by the competition model to be used.