Titania- and zirconia-supported gold particles of 1-5 nm site, prepared by various routes of synthesis, were employed in the partial hydrogenation of acrolein. In-depth characterization of their structural and electronic properties by electron microscopy, electron paramagnetic resonance, and optical absorption spectroscopy aimed at disclosing the nature of the active sites controlling the hydrogenation of C=O vs C=C bonds. The structural characteristics of the catalysts, as mean particle size, size distribution, and dispersion, distinctly depend on the synthesis applied and the oxide support used whereby the highest gold dispersion (D-Au = 0.78, Au/TiO2) results from a modified sol-gel technique. For extremely small gold particles on titania and zirconia (1.1 and 1.4 nm mean size), conduction electron spin resonance of the metal and paramagnetic F-centers (trapped electrons in oxygen vacancies) of the support were observed. Besides the influence of the surface geometry on the adsorption mode of the alpha,beta -unsaturated aldehyde, the marked structure sensitivity of the catalytic properties with decreasing particle size is attributed to the electron-donating character of paramagnetic F-centers forming electron-rich gold particles as active sites. The effect of structural and electronic properties due to the quantum size effect of sufficiently small gold particles on the partial hydrogenation is demonstrated.