The kinetic model of nitrogen oxides reduction by ammonia on vanadium-containing catalysts considering the dynamic character ammonia surface interaction with NOx and oxygen served as a base pf mathematical model of unsteady-state processes in the fixed catalyst bed. The theoretical analysis of the model helped to study the catalytic reactor behavior, when the stoichiometric ratio of reagents NOx/NH3 is disturbed. A distribution of adsorbed ammonia along the catalyst bed was studied as a function of time between the reverses of gas flow passing through the catalyst bed. The model allowed to describe qualitatively the high degrees (up to 99%) of gas conversion of NOx obtained using a large scale plant.