Abstract: A nickel/alumina catalyst, with an atomic ratio of nickel/aluminum between 10 and 2, an active nickel surface area between 70 and 150 m.sup.2 /g nickel and an average pore size, depending on the above atomic ratio, between 4 and 20 nanometers. Preferably the catalyst has a specific porous structure and the nickel/aluminum atomic ratio is between 10 and 4.A method is also disclosed for preparing the catalyst by a two step process involving precipitating nickel ions and adding during a second, so-called ageing step, a soluble aluminum compound. The catalyst is useful for hydrogenating unsaturated organic compounds in particular oils.