Abstract Ozone (O 3 ) and hydrogen peroxide (H 2 O 2 ) (the mixture is commonly called peroxone) have been analysed potentiometrically using Pt indicator electrode and I - / I 3 - redox couple as a probing potential buffer. The analysis is based on the measurement of the change of the open circuit potential of the indicator electrode as a result of the reaction of the oxidant mixture and I − . The large difference in the reaction rates of the oxidation of I − by the two oxidants enabled a selective and simultaneous analysis of both oxidants: the oxidation of I − by O 3 is instantaneous and results in a transient potential change, while that by H 2 O 2 is comparatively slow and it takes a few minutes for a steady state potential change to be attained. The experimental factors affecting the oxidation of I − by the two oxidants were examined for optimizing the selectivity. Further, the equation describing the time dependency of the potential change was derived by considering the rate constants of the reactions of I − and the oxidants and it was found to fit the experimental results.