The SCR of NO and NO decomposition were investigated over a V-W-O/Ti(Sn)O-2 catalyst on a Cr-Al steel monolith. The conversions of NO and NH3 over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH3 was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V10O31H12 (V-V) and V9WO31H12 (V-W) modelling (001) surfaces of vanadia and WO3-V2O5 solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W-N bond.