Hydrogenolysis of hexane, 2- and 3-methylpentanes, 2,3- and 2,2-dimethylbutanes and 2,2,3-trimethylbutane has been investigated on a 20 wt% Ni/SiO 2 catalyst in an attempt to understand the reasons why hydrogenolysis of alkanes leads almost exclusively to successive demethylations. However, besides demethylation, multiple hydrogenolysis and hydrogenolysis of internal bonds also occur, to a small extent, depending on the hydrocarbon structure. In contrast with observations on platinum, the activity of the catalyst is influenced little by the hydrocarbon structure especially when there is no quaternary carbon atom in the molecule. The selectivity in the primary CC bond rupture indicates that the CC bonds can be classified according to their increasing reactivity in the order C IC II > C IC III > C IC IV. A detailed analysis of the product distribution led to the conclusion that 1,3- and 1,4-diadsorbed species probably are not intermediates in the splitting of terminal CC bonds on nickel. On the contrary, 1, 2-diadsorbed species account very well for the selectivity in the hydrogenolysis of alkanes on this metal. Hence, the very high selectivity of nickel for successive demethylations of alkanes probably comes from its preference for the adsorption of primary carbon atoms and its ability to form 1,2-diadsorbed species.