The steady-and unsteady-state catalytic behaviour of Cu-MFI in the conversion of propane and NO in the presence of O-2 is reported, showing how the chemisorption and transformation of reactants may influence the surface reactivity. Various effects were observed : (i) a change in the surface reactivity and kinetics in going from low tn high concentrations nf NO nr propane (ii) the transformation of NO to N-2 and N2O promoted at low temperature (250 degrees C) by oxygen in the absence of hydrocarbon, (iii) the influence of NO over the surface reactivity of the catalyst in the conversion of propane and (iv) the influence of surface precoverage with oxidized nitrogen oxides (NxOy) or carboxylate species on the catalyst transient reactivity in the reduction of NO to N-2. In particular, Cu-MFI is initially more active when oxidized nitrogen oxides are present, suggesting that the active intermediate in the reduction of NO with propane is a complex formed by the reaction of nitrate with activated hydrocarbon. It is shown, however, that strongly bound oxidized nitrogen oxides may have also additional effects on the surface reactivity : (i) can promote the conversion of NO to N-2 and N2O in transient conditions and (ii) can give a partial inhibition of the surface reactivity blocking copper ions due to their strong chemisorption. Furthermore, it is shown that NO reacts faster with oxygen than hydrocarbon forming NxOy species which are then the oxidizing agent for the hydrocarbon. It is thus suggested that the surface reactivity of Cu-MFI in the reduction of NO with propane/oxygen depends on the surface population of nitrogen oxide adspecies which influence not only the surface reactivity, but also the pathway of hydrocarbon oxidation.