Experimental results of the investigations of the kinetic of selective reduction of nitrogen oxides in the kinetic region (powder of catalyst) and in the diffusion region (grains of catalyst) were described. It has been shown, that for compositions of reactive mixtures conforming to real exhaust gases, the rate of reaction can be described by the first order equation in relation to nitrogen oxides. It has been stated, that calculated parameters of kinetic equations, i.e. rate constants and effective coefficients of diffusion have the values well-founded physicochemically.