The behaviour of ammonia and oxygen over silica supported molybdena catalysts has been studied by means of thermal analysis techniques, such as thermo-gravimetry and heat-flow calorimetry. The composition of the reactants and products was determined by means of mass spectrometric analysis. Nitrogen- and hydrogen-containing species were observed to be present on the catalyst surface after reduction of the catalysts by ammonia and subsequent flushing in helium. Additional evidence for the presence of these species was provided by experiments in which labelled molecules were used as well as by means of X-ray photo-electron spectroscopy. The nitrogen- and hydrogen-containing species could be removed from the surface by O2. or NO. The reaction products were found to be N2 and H2O.