Several high surface area graphite-supported Ni catalysts were prepared and reduced by different methods. Samples were prepared by impregnation of the support and then submitted to traditional H-2 flow reduction at high temperature or to reduction with aqueous hydrazine at low temperature, and compared with a sample prepared by the deposition-precipitation method by means of the reduction of the dissolved Ni precursor by hydrazine at low temperature. The surface of these catalysts was characterised by means of CO adsorption microcalorimetry and by X-ray photoelectron spectroscopy (XPS), and a correlation of the results obtained with those on the selective hydrogenation of citral can be done. In this sense, only Ni-0 sites were detected at the surface of the Ni catalysts prepared by reduction with hydrogen leading to high catalytic activities and selectivity towards citronellal formation, while Ni oxidised species are also present (and are major) for catalysts prepared by reduction with hydrazine, what leads to an enhanced selectivity towards the formation of the unsaturated alcohols, geraniol and nerol. The oxidation of these latter catalysts seems to be only superficial, as suggested by the absence of H-2 consumption peaks in the TPR experiments. The presence of Ni-0 sites over these latter catalysts was not detected by XPS, thus our study proves that CO adsorption microcalorimetry is a very powerful tool for surface characterisation of heterogeneous catalysts. (c) 2006 Elsevier B.V. All rights reserved.