Ozonation characteristics of synthetic Procaine Penicillin G (PPG) formulation effluent were investigated in a semibatch ozone reactor at different pH (3, 7 and 12), ozone feed rates (600-2600mgh(-1)) and COD values (200-600mgl(-1)). Ozonation of aqueous PPG effluent resulted in 37 (82)% COD removal after 60 (120) min ozonation when the reaction pH was kept constant at pH = 7.900mgl(-1) (corresponding to 50% of the total introduced) ozone was absorbed during a reaction period of I h. The effects of increasing the applied ozone dose and the initial COD on the COD abatement rates of PPG effluent were also studied. Results have indicated that increasing the ozone dose and decreasing the COD content both have positive effects on COD removal rates. The significant contribution of the free radical ((OH)-O-center dot) reaction pathway to PPG ozonation could be traced using tert-butyl alcohol as the (OH)-O-center dot probe compound at varying concentrations. The bimolecular reaction rate constants for the direct reaction of PPG with ozone were found as 152 and 2404 M-1 h(-1) at pH = 3 and 7, respectively, using the gas phase ozone partial pressures determined from of the outlet gas stream analysis. It could be demonstrated that ozone decomposition to free radicals being triggered by increasing the pH from 3 to 7 is essential for the rate enhancement of PPG effluent ozonation. (c) 2004 Elsevier Ltd. All rights reserved.