Two series of nickel catalysts supported on sepiolite have been prepared by impregnation and precipitation procedures, and the resultant materials characterised in their reduced states. FTIR of adsorbed CO and X-ray photoelectron spectroscopy (XPS) were used to study the surfaces of the materials and the degree of reduction was calculated gravimetrically. The metal dispersion/particle size was calculated from hydrogen adsorption isotherms at 298 K, and the hydrogenation of benzene used as a test reaction of the metal sites. No correlation between activity and metal dispersion was found and turnover frequencies indicate an apparent structure sensitivity of the reaction in addition to an apparent influence of the preparation method. However, similarity between the form of plots of turnover frequency–particle size and degree of reduction–particle size, suggests that the apparent structure sensitivity is the result of the presence of unreduced Ni in the catalyst surfaces. It is proposed that the presence of this unreduced Ni acts as a diluent in the Ni surface, thereby reducing the size of Ni ensembles.