A series of well-defined nickel, copper and nickel-copper molybdenum complex compounds isomorphous of the ammonium triammine tetranickel pentamolybdate were prepared. The characterisation of the catalysts obtained by reduction in hydrogen of these precursors is difficult but the use of the chemisorptive decomposition of nitrous oxide together with the reactions of benzene hydrogenation and propanol decomposition make possible an estimation of the surface atomic copper-nickel ratio. Although the precursors are very homogeneous, an enrichment in copper of the surface metallic phase obtained after reduction is observed.