We have studied nickel classical catalysts supported on Al2O3, SiO2 and Nb2O5 oxides, using simple (SIM) or EDTA-double impregnation (DIM) methods. Non-classical catalysts were obtained by pretreatment of the SIM or DIM nickel precursors in aqueous hydrazine at 353 K. The catalysts were characterized by H-2-adsorption, H-2-TPR, isopropanol decomposition and FTIR measurements. They have been tested in the gas phase hydrogenation of benzene. The results obtained show that the support surface acidity order was: SiO2 < Al2O3 < Nb2O5. The catalysts exhibited various nickel species due to the existence of various metal-support interaction strengths. As a consequence, the reducibility, surface or hydrogenating properties changed as a function of the nature of the support or method of preparation. The best performances were obtained with the silica support which seemed to interact less with the nickel. The metal-support interactions are highest when niobium oxide is used as the matrix. Ni impregnated on niobia was not active at all, due to very strong metal-support interactions. The interaction of aqueous hydrazine with the catalyst in the reduction conditions was examined. It was shown that hydrazine formed stable complexes with nickel, whereas it adsorbed on the support with a strength depending on the nature of the oxide. (c) 2005 Elsevier B.V. All rights reserved.