The ubiquitous presence of trace pharmaceutical compounds in the environment is a significant concern. While the implications of these compounds on ecosystems and human health are being determined, there has been increasing interest in their treatment such as by tertiary processes at sources and wastewater treatment facilities to arrest further release to the environment. We have examined the degradation of sulfonamide and macrolide antibiotics in a spiked water and a pharmaceutical wastewater by ozonation under varied conditions such as concentration, contact time, pH, and H2O2/O-3 mole ratio. The results show faster removal kinetics for sulfonamides containing the aromatic ring than for macrolides built of mostly saturated hydrocarbon structure, and that complete removal of all is achieved within 20 min of ozonation at the application rate of 0.17 g O-3/min. Degradation of contaminants containing unsaturated C-C bonds occurs faster at low pH, consistent with O-3 being the predominant oxidant and its aqueous concentration being higher at low pH. Degradation of erythromycin having a fully saturated structure is slower and more effective at higher pH or with added H2O2, both consistent with the enhanced production of OH radical under such conditions that contributes to removal of the saturated compound. Low pH favors degradation via molecular O-3 while high pH via OH radical; the optimal pH thus depends on target compounds being treated, and buffered pH at 7 facilitates removal of all tested compounds. The addition of H2O2 to ozonation abets contaminant removal, and at mole ratio of H2O2/O-3 = 5 it attains the highest degradation speed for all contaminants. However, a large excess of added H2O2 results in reduced or no benefits relative to O-3 alone. Thus, only a small dose of H2O2 is desirable when widely disparate compounds are treated by ozonation. (C) 2009 Elsevier B.V. All rights reserved.