The ozonation of p-chlorophenol (CHP) in aqueous solution has been studied in the pH range 2.0-8.0, in the presence of tert-butyl alcohol, which prevents the activation of the radical mechanism of oxidation. Results indicate that the pH influences the system reactivity and that only a partial chlorine release is observed for lengthy ozonation too, after the complete substrate disappearance. For adopted experimental conditions the oxidation process develops under a quasi-diffusional regime of absorption with reaction, a transition domain between kinetic and diffusional regimes in which ozone and dissolved substances react exclusively in the liquid film. A proper mathematical model has been developed and used to simulate the system behaviour