Combined kinetic and potentiometric measurements have been used in the investigation of partial oxidations of organic compounds on oxidic catalysts of increasing complexity. The response of the catalyst to variations of the gas phase composition consists of abrupt phase transitions in the case of propene oxidation on copper oxides and to the existence of a large 'concentration window' in the case of acrolein oxidation on a multicomponent oxide where the oxidation state of the catalyst is kept constant at values favourable for high selectivity. Solid electrolyte potentiometry is shown to be an excellent tool to probe in situ the oxidation state of catalysts under working conditions.