The effect of dissolved ozone on the electrochemical behaviour of heat exchanger structural materials (carbon steel, stainless steel, copper, 70:30 copper-nickel, aluminium brass and titanium grade 1) was studied to evaluate the possibility of using ozone as sole biocide in cooling water treatment. With this purpose, voltammetric and open circuit potential (OCP) against time measurements at different ozone concentrations between 0.1 and 1.2 ppm were made. Results show different electrochemical responses according to the metal characteristics and the solution composition. First, the passivity of titanium and stainless steel was not affected by ozone. A linear OCP against log time relationship was found for titanium, suggesting the growth of a barrier film in both O-2 and O-2/O-3 solutions. Mild steel does not passivate in synthetic cooling water either with O-2 or O-2/O-3 in the solution. In the presence of ozone the breakdown of passivity is facilitated and makes the repassivation difficult. Ozone enhances the dissolution of Cu2O and the formation of Cu(II) species leading to less protective films. Both processes are strongly influenced by the pH. Finally, the dissolution of aluminium brass is higher than that of copper or copper-nickel.