The partial oxidation of propene over Au-Ag alloy (14 mol%) and monometallic Ag supported over a dense Solid Electrolyte Membrane Reactor (SEMR) was studied and compared under electro-catalytic reaction conditions. Two partial oxidation products, namely acrolein and 1,5-hexadiene, in addition to carbon dioxide were detected. It was observed that the Au-Ag alloy was more selective towards acrolein formation, possibly promoted by oxygen adsorbed molecularly on exposed Ag surfaces whereas 1,5-hexadiene formation is promoted by oxygen species adsorbed atomically on silver surfaces. Taking into account that gold has no ability to dissociate molecular oxygen, and that acrolein and 1,5-hexadiene were not observed under open circuit conditions it is suggested that oxide ions are reduced at the triple phase boundary to form an active oxygen species which spills onto the catalyst surface to react with adsorbed propene. Differences in selectivity can only be explained by a difference in oxygen type and surface population of the catalysts based, for example, on a sufficiently large change in the work function of the catalysts. (C) 2008 Elsevier B.V. All rights reserved.