The effect of the ruthenium precursor (chlorinated or chlorine-free) on the properties and the catalytic behaviour of Ru/ZnO catalysts in the vapour phase hydrogenation of crotonalclehyde (2-butenal) has been studied. Two catalysts were prepared, using ruthenium (III) chloride (Ru(Cl)/ZnO) and ruthenium (III) acetylacetonate (Ru(A)/ZnO) as the metal precursors. The catalysts have been characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), H-2 temperature-programmed desorption (H-2-TPD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), and their catalytic behaviour has been evaluated in the title reaction after reduction at low (473 K) and high (623 K) temperatures. Ru(Cl)/ZnO showed better catalytic behaviour after both reduction treatments, what has been explained on the basis of a well controlled metal-support interaction. For Ru(A)/ZnO this interaction is too high. resulting in a dramatic lost of active sites that yields worse catalytic behaviour. It is important to remark that both catalysts were very active and selective to the desired product, the unsaturated alcohol (crotyl alcohol, 2-buten-1-o1). (C) 2009 Elsevier B.V. All rights reserved.