The activity of Ni supported on gamma-Al2O3, SiO2, TiO2, MgO, ZSM-5 and ZrO2 for the selective conversion of NH3 to N-2 in the presence of excess O-2 has been examined. The results show that Ni/Al2O3 and pure, acidic gamma-Al2O3 are the most selective catalysts, with, respectively, N-2 selectivities of similar to 90% at 550 degrees C and ca, 95% at 625 degrees C. Characterisation by TPR and XRD indicates that difficult-to-reduce Ni/Al2O3 catalysts are the most selective. Ni loadings below 10% give rise to a spinel phase, NiAl2O4, which appears to be responsible for the enhanced catalytic behaviour. For Ni loadings above 10%, the selectivity gradually decreases due to the presence of free NiO crystals on the modified support. Other metal oxides (Mn, Fe, Co, Mg, Zn) which are soluble in gamma-Al2O3 have also been investigated. Mn/Al2O3 and Fe/Al2O3 are more active than Ni/Al2O3, possibly because of the enhanced redox properties. Full conversion is obtained with Mn/Al2O3 at 300 degrees C and with Fe/Al2O3 at 500 degrees C, with the corresponding maximum selectivities to N-2 being ca. 100 and ca. >90%, respectively. N2O is formed with a selectivity of ca. 20% at low temperature on Mn/Al2O3.