The surface sites on titania-supported chromia (CrO(x)/TiO2), which are relevant for the selective catalytic reduction (SCR) of NO by NH3, and the undesired formation of N2O have been investigated using temperature-programmed desorption (TPD) combined with in situ diffuse reflectance FTIR spectroscopy (DRIFT). The combined TPD and DRIFT measurements carried out on the differently pretreated (reduced, oxidised, exposed to SCR conditions) CrO(x)/TiO2 indicate that Bronsted-bound ammonia is crucial for the SCR reaction. N2O is formed at temperatures above 450 K by direct oxidation of Lewis-bound NH3 with oxygen present in the SCR feed gas. TPD measurements carried out with CrO(x)/TiO2 previously loaded with NO and NH3, respectively, showed that neither oxidised nor reduced CrO(x)/TiO2 Convert NO in the absence of NH3 at the temperatures used for SCR. Adsorbed NO has been identified in various forms, including nitrate (1613 and 1517 cm-1) and a Cr-ONO species (1458 cm1). When adsorbed as a single reactant, ammonia desorbs molecularly from the prereduced catalyst and undergoes direct oxidation on a preoxidised catalyst leading to N2O and H2O at temperatures higher than about 350 K. At temperatures above 450 K, the direct oxidation of ammonia yielding N2, becomes significant. In the absence of NO, direct oxidation of NH3 to N2 and N2O occurs via Lewis-bound ammonia. In the presence of oxygen, ammonia is oxidised to NO above about 500 K. TPD experiments with CrO(x)/TiO2, previously either reduced or oxidised and then exposed to the SCR feed gas, showed that the surface reduced with ammonia is inactive for the desired selective reaction between NO and NH3, indicating that a partially oxidised state of the surface must be maintained for SCR; this observation confirms the crucial role of the oxygen in the SCR feed gas. The studies indicate that Bronsted-bound ammonia, characterised by bands at 1662 and 1425 cm-1, enhances the adsorption of NO and is involved in the selective catalytic reduction of NO to N2.