In the so-called photocatalytic ozonation (PO) synergistic effects between photocatalysis and ozonation enhance the rate of oxidation of organic compounds, therefore this integrated process is very promising for use in water treatment. In the present work, it is demonstrated that the improvement that can be obtained by the integrated process in comparison with the uncoupled processes depends on the relative weight between photocatalysis and ozonation. In this respect, it has been found that a relatively low rate of photocatalysis (0.3 times the rate of ozonation) is sufficient to get the highest synergy and to maximize the performances in the oxidation of formate, the adopted model organic compound. The benefits of the coupling of photocatalysis to ozonation are not limited to the increase of the oxidation rate, but include also the control of the ozonation by-product of main concern (bromate). The effect of the pH, of the presence of oxidizable organic compounds, of the presence of nitrogen on the formation or on the reduction of bromate are experimentally studied and discussed. The obtained results have relevance for the practical applications of PO, since on these bases it is possible to identify proper operating conditions. (C) 2014 Elsevier B.V. All rights reserved. The authors wish to thank MIUR (Rome) for economical support (Project: PON02_00153_2849085).