The changes of the surface properties of two bulk Ni (one commercial Raney-Ni and one prepared crystalline Ni) and two Ni-MgO catalysts were studied after the hydrogenation of adiponitrile in the gas phase at atmospheric pressure for 216 h at two different reaction conditions. Catalysts before and after the catalytic reaction were characterized by different techniques. Raney-Ni and Ni-MgO catalysts show an evolution of activity with the reaction time until reaching constant conversion and selectivity values. The metallic area decreases after reaction for all catalysts. Interestingly, this decrease is lower and the surface area slightly increases when using reaction conditions of high space velocity, which favour the desorption of condensation products and results in a certain disagglomeration of particles. The high hydrogenation ability of Raney-Ni and the basicity of Ni-MgO catalysts allow them to achieve much higher conversions than the bulk Ni catalyst prepared. The presence of defined octahedral particles, observed for the crystalline Ni and Ni-MgO catalysts by SEM, has been related to the constant high value of selectivity to 6-aminohexanenitrile (around 80%) maintained after 216 h of reaction at less hydrogenating conditions. (TM) 2004 Elsevier B.V. All rights reserved.