It is shown how the deactivation of a diesel SCR (Selective Catalytic Reduction) catalyst by compounds in the exhaust gases influences the kinetics of the catalyst. Results are given for the fresh (4.56 % V2O5/TiO2) catalyst and the ones used in the rig for 890 h and 2299 h. The reactions of 700 ppm NH3 and 2 % O2 in Helium yielded N2, N2O, and NO at increasing temperatures. Simulations were performed with COMSOL Multiphysics using a 3-D model of the catalyst system. The experimental values of the products N2, N2O, and NO were very nicely fitted by the kinetic model used. All three ammonia oxidation reaction rates were of the first order in the concentration of NH3. A preliminary study using non-isothermal conditions showed the maximal temperature increase to be 0.15 K. Thus, further simulations were done with an isothermal model. The deactivation reduces the pre-exponential factors and the activation energies for the formation of N2, N2O, and NO. The formation of N2 is not substantially influenced by deactivation. The changes in the kinetics of the catalytic NH3 oxidation by deactivation is reported for the first time in the present study.