Efficiency of the ozone (O$_3$)/Activated Carbon (AC) process to remove phthalates was studied by investigating the degradation kinetics of diethylphthalate (DEP) chosen as model pollutant. The influence of pH was determined by performing experiments at five pH values (2.5 < pH < 7.2). The kinetic contribution of radical mechanisms (δ$^{HO•}$) was estimated by using a radical scavenger (tert-butyl alcohol). Single ozonation of DEP was first performed. Pseudo-first order modelling shows that the reaction depends on pH, with kinetic constants varying from 0.0036 min$^{−1}$ (pH = 2.5) to 0.6129 min$^{− }$1 (pH = 7.2). Ozonation was then performed in the presence of a commercial AC (Pica L27), whose chemical and textural properties had been previously determined. Pseudo-first-order modelling shows a significant increase in DEP degradation kinetics: kinetic constants rose from 0.290 min$^{−1}$ (pH = 2.5) to 0.866 min$^{−1}$ (pH = 7.2). Estimation of the kinetic contribution of heterogeneous reactions to DEP removal (δ$^{hetero}$) shows that in acidic conditions, reactions are essentially located on the AC surface (δ$^{hetero}$ = 98.7% at pH = 2.5) and occur in the bulk liquid when pH increases (δ$^{hetero}$ = 29.2% at pH = 7.5). Lastly, estimation of the kinetic contribution of radical mechanisms (δ$^{HO•}$) shows that the increase in the DEP degradation kinetics is mainly due to radical reactions, with δ$^{HO•}$ increasing from 87.9% (pH = 2.5) to 92.0% (pH = 7.5). These radical reactions may be promoted by deprotonated acid groups present on the AC surface.