In view of the importance for CoxOy-MoO3/gamma-Al2O3 hydrodesulphurization (HDS) catalysts, the reactivity of cobalt oxide layers towards cobalt aluminate formation was investigated on both MoO3-covered and bare gamma-Al2O3 substrates. Co3O4/MoO3/gamma-Al2O3 and Co3O4/gamma Al2O3 systems were prepared by vapour-deposition of MoO3 (12 x 10(15) Mo atoms/cm(2)) and Co (400 x 10(15) Co atoms/cm(2)) layers onto a gamma-Al2O3 substrate, followed by oxidation of the Co layer to Co3O4. After annealing at 800 degrees C for 40 h, the interfacial reaction to cobalt aluminate was assessed using Rutherford backscattering spectrometry. The presence of molybdenum oxide appeared to enhance cobalt aluminate formation. The Mo atoms, which spread out over the entire cobalt-containing layer, presumably caused a high defect density, which explains the observed higher reaction rate. The amount of MoO3 was much too low to stabilize all cobalt atoms by cobalt molybdate formation.