Certain Co/Al2O3 catalysts are active and stable for the selective catalytic reduction (SCR) of NOx by propene or propane. The addition of 30 ppm SO2 to the feed only mildly affects the catalytic performance. The effectiveness of Co/Al2O3 in the SCR process depends strongly on the Co loading, the calcination temperature, and the source of alumina. Characterization of the catalysts with UV-vis spectroscopy under ambient conditions, temperature-programmed reduction by H-2 (H-2-TPR), XRD, iodometric titration, and BET surface area measurements led to the proposal that there exist four different Co species : Co2+ ions in CoAl2O4, small Co3O4 particles that interact strongly with Al2O3, large Co3O4 particles, and dispersed surface Co2+ ions in octahedral coordination. The latter Co2+ species are likely the catalytically most active species. Their propensity to interact with NOx is reminiscent of that of Co ions in ZSM-5.