In a H-2 and O-2 mixture, the reaction of propene switches between hydrogenation and epoxidation over Au/Ti-based oxides. Reaction pathways are strongly dependent on the size of Au particles and on the presence of alkalis. The hydrogenation prevails over Au clusters smaller than 2.0 nm (in the absence of alkalis) and over Au nanoparticles larger than 5.0 nm. Alkali contamination can switch hydrogenation to epoxidation over Au clusters and promote epoxidation over Au nanoparticles with diameters of 2.0-5.0 nm. It was also confirmed that the hydrogenation of propene is enhanced by the addition of a small amount of O-2. Switching of product by alkalis and enhancement of hydrogenation by oxygen are interpreted by parallel and competitive reaction pathways. (C) 2011 Elsevier Inc. All rights reserved.