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 alpha-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 precursor in hydrogen, the chromia material remains amorphous for temperatures below about 420 degrees C. Samples heated above ca. 470 degrees C are highly crystalline. The crystallinity of the chromia materials 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. Kinetic 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 alpha-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 degrees C, whereas for alpha-Cr2O3, the effect varies with reaction temperature. For amorphous chromia the addition of water substantially decreases the formation of nitrous oxide.