The hydrogenation of cinnamaldehyde was studied over a series of ZrO2-supported monometallic and bimetallic Ru catalysts. High surface area zirconia was prepared by precipitation of ZrOCl2 in an abkaline medium. Depending on the thermal treatments, amorphous, tetragonal or monoclinic ZrO2 phases were obtained. The metal catalysts were characterized by H-2 chemisorption, transmission electron microscopy, X-ray diffraction, and infrared spectroscopy of adsorbed CO. When compared at very similar sizes of the metal particles, Ru/ZrO2 catalysts are much more selective to cinnamyl alcohol than Ru/AlO3. Moreover, among the different Ru/ZrO2 samples, that supported on amorphous ZrO2 exhibits the highest selectivity to the unsaturated alcohol. It is proposed that, during deposition of Ru(acac)(3), the acetylacetone liberated extracts some zirconium as Zr(acac)(4). During the reduction step, this Zr acetylacetonate decomposes and covers the Ru atoms, or forms a bimetallic Ru-Zr-n+ type of complex. The presence of this adspecies on the Ru particles is in line with the low H-2 uptake of Ru/ZrO2(amorphous) with respect to the size of the Ru particles (dp approximate to 1.8 nm). This kind of zirconium species is not present on Ru particles supported on crystalline ZrO2; sizes of Ru particles deduced from TEM pictures and H-2 chemisorption are in good agreement. The increase of cinnamyl alcohol selectivity could be due to interfacial Ru-Zr-n+ sites at the periphery of the particles. On all these Ru/ZrO2 catalysts the presence of mixed Ru-Zr-n+ sites decreases the strength of the C=O bond, making easier the attack by hydrogen. This hypothesis is supported by a broad peak of CO adsorbed on Ru/ZrO2 (tetragonal) at low frequency (v(co) x 1860 cm(-1)).