Various solids with a hydrotalcite-like structure and containing Ni2+/Mg2+/Al3+ cations in different amounts were prepared. These solids were calcined at 623 K and reduced at 723 K to give Ni/Mg(Al)O catalysts, with metallic Ni particles between 7 and 9 nm. The properties of Ni and the support for the adsorption of monoethylamine, acetonitrile, CO, and H-2 were evaluated by calorimetric and temperature-programmed desorption experiments. These catalysts were tested in the gas phase hydrogenation of acetonitrile between 350 and 450 K and with a H-2/CH3CN molar ratio of 6.75. The main product is monoethylamine (MEA). Secondary products are N-ethylethylimine at low conversion and diethylamine and triethylamine at high conversion. The MEA selectivity and, to a much lesser extent, the rate depend on the Mg/(Mg + Ni) ratio. An optimal Mg content (Mg/(Mg + Ni) approximate to 0.23) exists for which the MEA selectivity goes through a maximum value of 92.6% at 99% CH3CN conversion. The occurrence of this optimal Mg content is due to a compromise between the reducibility of Ni and the acido-basicity of the material, both decreasing with the introduction of Mg. The acid sites are partly responsible for the formation of secondary products. The heat of MEA adsorption, which is 40 kJ mol(-1) lower for the Mg-containing sample (Mg/(Mg + Ni) approximate to 0.23) than for the Mg-free sample, provides evidence of a decrease in the acidity for the composite Ni/Mg(Al)O. This is further confirmed by the disappearance of the high temperature peaks of MEA desorption for the latter sample.