This study addresses a kinetic investigation of NH3-SCR on commercial SCR catalysts with composition V2O5/MoO3/TiO2. Objective of the study is an improved kinetic description of the reaction, based on the recognition of the redox nature of the V-site; which calls for an understanding of the effect of O-2. At this scope, an extensive experimental investigation over a powdered catalyst was performed at largely varying concentrations of O-2 (from 0.06 to 8%) both at high and at low NO and NH3 concentration. It was found that the increase of the O-2 feed content promoted NO conversion with asymptotic trend: the promotion was important in the range 0.06-3%, but became moderate in the range 3-8%. Unexpectedly, the effect of O-2 was equally important at high NO and NH3 concentration as well as at low NO and NH3 concentration (where a kinetic control from the reduction step and thus a negligible role of the oxidation step were expected). The kinetic analysis revealed that the observed experimental trends can be described by a Mars-van Krevelen rate expression assuming that the rate of re-oxidation depends on NO concentration as well as the rate of reduction does. A molecular rate equation which incorporates a simple linear dependence on NO concentration in the re-oxidation kinetics fully describes the whole bulk of data and is suitable to engineering purposes. The existence of an NH3 inhibiting effect was also included. The global rate expression was successfully validated against independent pilot-scale experiments on slabs.