Effect of the Ni precursor and of the copper addition on the properties of Ni/KL-supported catalysts was studied by TPR, H-2 chemisorption, N-2 adsorption, CO-FTIR and CO adsorption microcalorimetry. Characterization measurements of catalysts prepared from nickel (II) nitrate hexahydrate, nickel (II) acetylacetonate and nickel (II) phthalocyanine, respectively, show different nickel species distribution in the zeolite, with electron rich metal particles inside the lattice. For NiCu/KL bimetallic catalysts, combined studies by CO-FTIR and by CO adsorption microcalorimetry, together with the TPR results, provide evidence of formation of bimetallic nanoparticles with alloy type structure. The contribution of this species to the surface composition of the catalysts depends on the respective Ni/Cu atomic ratio. Encapsulation of part of the metal components in the zeolite channels and formation of segregated copper particles were also found. Results in the hydrogenation of citral at 323 K in the liquid phase evidence a positive relationship of the catalytic activity with the population of electron rich metal species on the monometallic catalysts. Copper inhibits the hydrogenation activity of nickel and modifies the structure of the exposed nickel surface, which affects the stability of the catalysts and the selectivity for the competitive hydrogenation of conjugated C=C and C=O bonds of the citral molecule. (C) 2008 Elsevier B.V. All rights reserved.