This research forwards the evaluating of the efficiency of nanosized ZnO in the application of the catalytic ozonation, determines the reaction kinetics of ozone with the nanosized ZnO, and delineates the characteristics of the decomposition of para-Chlorobenzoic acid (pCBA) and ozone in catalytic ozonation using the nanosized ZnO particles. It was found that the nanosized ZnO enhanced the degradation of ozone and the catalytic ozonation on the surface of the nanosized ZnO significantly enhanced the degradation of pCBA. In catalytic reactions the degradation of pCBA followed two-stage kinetics, initial rapid removal phase (Phase 1) and a slower decomposition phase (Phase 11). The total degradation rate of pCBA was well matched with the initial removal of pCBA in Phase I. The R-ct values representing the ratio of hydroxyl radicals [(OH)-O-.] and ozone 1031 were found to increase with an increased concentration of the nanosized ZnO indicating the enhanced transformation of ozone into (OH)-O-.. (c) 2005 Elsevier B.V. All rights reserved.