The effects of preparation procedure, oxygen concentration and water on the reaction of nitric oxide and ammonia over chromia catalysts prepared by thermal treatment of chromium hydroxide have been determined. Analyses were performed using a combination of mass spectrometry, Fourier transform infrared spectroscopy and gas chromatography. Thermogravimetric and differential thermal analysis techniques show that the temperature at which transformation of amorphous chromia to the α-Cr2O3 phase occurs is dependent on the gaseous atmosphere during the heat-treatment process, and increases in the order oxygen, nitrogen and hydrogen. When heating the catalyst precursorin hydrogen, the chromia material remains amorphous for temperatures below about 420°C. Samples heated above ca. 470°C are highly crystalline. The crystallinity of the chromia material is the most significant factor in determining the activity, and selectivity, of the resulting catalysts for the NO/NH3 reaction. Amorphous chromia catalysts prepared with a two order-of-magnitude difference in ammonia addition rate show similar performances. Kenetic orders in oxygen to produce both nitrogen and nitrous oxide have been determined. Amorphous chromia shows a much higher order in oxygen for nitrogen formation than does α-Cr2O3. However, for nitrous oxide formation, the orders are approximately the same for both morphologies. Addition of 1.5% H2O vapour decreases nitric oxide and ammonia conversions over amorphous chromia for all temperatures below 275°C, whereas for α-Cr2O3, the effect varies with reaction temperature. For amorphous chromia the addition of water substantially decreases the formation of nitrous oxide.