The partial oxidation of propene over silver has been carried out at two temperatures (450 and 500 degreesC) under highly reducing conditions (small oxygen/propene ratio). Two partial oxidation products were detected, namely, acrolein and 1,5-hexadiene, in addition to the total oxidation product CO2. Electrochemical oxygen supply to the catalyst through a solid electrolyte membrane induces the formation of 1,5-hexadiene, whilst gas-phase oxygen provision yields mainly acrolein. Total oxidation product selectivity is not affected. It is suggested that different surface oxygen species (dependent on the means of oxygen supply) may be responsible for the observed behaviour. (C) 2004 Elsevier B.V. All rights reserved.