The surface properties of Pt-Re catalytic nanoparticles supported on carbon following exposure to a hydrogen reducing environment and subsequent hydrothermal conditions have been studied using in situ X-ray photoelectron spectroscopy (XPS) and ammonia temperature-programmed desorption (TPD). These properties have been correlated with the catalyst selectivity for the aqueous-phase reforming of glycerol. We show that Pt in reduced Pt-Re/C becomes electron deficient, and a fraction of the Re becomes oxidized when the catalyst is subsequently exposed to hydrothermal reaction conditions. Oxidation of Pt-Re generates surface acidity, which drastically affects the reaction pathways. The acid site concentration, but not acid site strength, increases with Re loading. This acidity increase with Re addition favors C-O over C-C cleavage, which results in higher selectivity to liquid products and alkanes at the expense of hydrogen selectivity. We propose a model for the Pt-Re active site and the origin of acidity enhanced by the addition of Re. (C) 2011 Elsevier Inc. All rights reserved.