Hydrogenation of benzene was used to probe the activity of Ni/Al2O3 catalysts with metal content in the 0.4-3 wt% range prepared with Ni acetylacetonate as a precursor. Average particle sizes in the 1.9-8 nm range were determined from hydrogen chemisorption measurements depending on the nickel content and reduction temperature. A preliminary decomposition of the organic ligands in air at 250 degreesC followed by reduction markedly increased the activity relative to a direct reduction. Treatments at higher temperatures had a detrimental influence on catalyst efficiency. Catalysts reduced at 350-400 degreesC were more active than those reduced at 450 degreesC and above, with the most active catalysts having nickel content of 1 and 1.6%. The dependency of the turnover frequency on particle size suggested that the reaction was structure sensitive for particle sizes below 4 nm and insensitive above this size. The fraction of unreduced nickel also had an effect on the activity. From a tentative comparison of the turnover frequencies (TOF) with values reported in the literature, it may be inferred that Ni(acac)(2) is a rather valuable precursor in preparing very efficient and stable catalysts with low nickel content.