Alumina-supported gold catalysts (Au/Al2O3) prepared under various conditions were observed on an atomic scale using aberration-corrected scanning transmission electron microscopy (Cs-STEM). The Au particles, including single Au atoms, on Cs-STEM images were extracted using morphological image processing, and the size distributions for reaction site candidates, such as the corner, edge, and surface of Au clusters, in a unit weight of Au/Al2O3 catalyst were calculated. Based on the size distributions for these reaction site candidates, we inspected the effective sites and sizes of Au catalysts for the hydrogenation of aldehydes (5-hydroxymethylfurfural and benzaldehyde) and a nitro compound (nitrobenzene). The results suggested that the corner atoms are a key site for the hydrogenation of aldehydes and the nitro compound because the dissociative adsorption of H-2 on corner atoms controls the reaction rate. Meanwhile, the effective size for the aldehydes was smaller than that for the nitro compound due to their different adsorption sites: while the aldehydes are adsorbed on the corners of Au clusters, the nitro group is adsorbed on their edges. The adsorption of the reactants was supported by reaction kinetics and X-ray absorption spectroscopy analyses. The specific active size for hydrogenation reactions was also investigated based on the size distributions. (C) 2015 Elsevier Inc. All rights reserved.