We investigated the influence of Pt particle size and the addition of Re in carbon-supported catalysts in the aqueous phase reforming (APR) of glycerol. Pt nanoparticles with averaged sizes of 1.2, 1.6, 2 and 4.2 nm were obtained by reduction at increasing temperatures of Pt/C. Gas-phase water–gas shift (WGS) and acetaldehyde decomposition reactions were used to separately study important reaction steps in the APR mechanism. The APR H2 turnover frequency, which was highest for 2 nm Pt particles, correlates with activity trends in the WGS reaction. The decarbonylation rate (acetaldehyde decomposition) is also highest for intermediate Pt particle size. Pt–Re nanoparticles were prepared by in situ catalytic reduction of HReO4 on Pt/C followed by reduction at 300 °C. The APR activity after Re addition on Pt/C increased for Pt/C–Re having initial the smallest particles, but decreased for the other larger-sized Pt/C–Re catalysts. The bimetallic catalysts were much more active in the WGS reaction than their Pt-only parents. An increase in the reduction temperature to 600 °C resulted in a further increase of the WGS activities. The different ways Re addition affects APR and WGS activities of Pt/C catalysts relate to different surface site requirements for these reactions. The presence of isolated Pt atoms, whose fraction increases when the alloys are reduced at higher temperatures, is sufficient to bring about the synergetic effect between Pt and Re for the WGS reaction. For APR, on the other hand, the presence of surface ensembles of Pt atoms is essential for obtaining high CO and CC bond breaking rates.