A systematic investigation of the influence of Ti/[IrO2-Nb2O5] electrode composition ([IrO2] = 40, 45 and 50 mol%) on electrochemical ozone production (EOP), was conducted in 3.0 mol dm(-3) H2SO4 in the presence and absence of 0.03 mol dm(-3) KPF6. "In situ" characterisation revealed all oxide layer presented similar structures, except for the 50 mol% IrO2 nominal composition which showed a higher porosity/roughness. The introduction of KPF6 in the electrolyte resulted in an inhibition of the oxygen evolution reaction (OER) at high current densities, improving ozone generation efficiency at i > 0.4 A cm(-2), while reducing overpotential for OER. When normalised for the area, the ozone current efficiency presented a good performance of the system. However, improvement of the electrode service life is necessary in order to support the drastic conditions observed during EOP. (C) 2004 Elsevier Ltd. All rights reserved.