The effect of the support (here silica and silica-alumina) and of the composition of Ni-based hydrogenation catalysts, elaborated by sol-gel method, on their physico-chemical characteristics and on their activity and selectivity (towards ethylene) for acetylene hydrogenation has been studied with the help of temperature programmed reduction (TPR) and oxidation (TPO), X-ray photoelectron spectroscopy (XPS) and reaction tests (acetylene to ethylene hydrogenation). These techniques have shown the existence of two different types of sites. Firstly, the hydrogenolytic (naked) metallic centres, corresponding to nickel without (or little) interaction with the support, are responsible for a large part of the coke and are active for side-reactions. Secondly, the hydrogenating sites correspond to nickel in interaction with the support and are active for the main reaction. Three types of coke have been detected: filaments, amorphous coke, and partially hydrogenated carbons strongly adsorbed on the surface. The latter are facilitated by the acidity of the silica-alumina. Because the metal-support interactions related to a better dispersion of reduced nickel, silica-alumina-based catalysts produce a much smaller amount of coke than silica-based catalysts. (C) 2003 Elsevier B.V. All rights reserved.