Cobalt sulphated-ZrO2 catalysts were prepared by impregnation of ZrO2 with aqueous solutions of CoSO4, or impregnation of sulphated-ZrO2 with toluene solutions of Co(acetylacetonate)(2). NO reduction with C3H6 in the presence of excess O-2 was studied in a flow apparatus fed by a reactant mixture of NO:C3H6:O-2 = 4000:2000:20,000 ppm in He. Diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) results showed that the presence of sulphates prevented the formation Of Co3O4. XPS indicated the presence of Co(II) alone. Irrespective of the preparation method, Fourier transformed infrared (FTIR) showed that catalysts with the same sulphate content had the same covalent sulphates. As the sulphate content in the sample increased, FTIR with CO or NO showed that the Lewis acid-strength of Co(II) increased, whereas the reducibility and heterogeneity of Co(II) decreased. Sulphated samples with a cobalt content higher than 2 atoms nm(-2) were far more active and selective than the corresponding unsulphated CoOx/ZrO2. The presence of sulphates had a weaker effect on the catalytic behaviour of samples with lower Co content (<2 atoms nm(-2)). We conclude that cobalt and sulphate co-operate in determining the catalytic activity and selectivity of cobalt sulphated-ZrO2 catalysts. (C) 2003 Elsevier Science B.V. All rights reserved.