The steady-state kinetics of the selective catalytic reduction (SCR) of nitrogen oxides (NO and NO2) with ammonia over a commercial iron zeolite catalyst were studied in the temperature range of 250 to 450 degrees C using an integral tubular reactor. Special attention was paid to the stoichiometric ratio of the conversion of ammonia and nitrogen oxides. For this purpose, both systematic SCR measurements at different feed gas compositions and independent studies of the catalytic oxidation of ammonia in the absence of NO and NO2 were carried out. Under all reaction conditions, a considerable deviation from the expected 1:1 stoichiometry was observed. The steady-state kinetics of the reacting system could be described by global Langmuir-Hinshelwood-type rate equations for standard SCR, fast SCR, NO/NO2 equilibrium and NH3 oxidation. For the correct calculation of the ammonia oxidation it was necessary to include two terms. A first one describing the reaction of NH3 with O-2, which becomes important at temperatures above 400 degrees C, and a second rate equation which is not only proportional to NH3 and O-2 but also to the NO concentration and which is of particular relevance at low reaction temperatures. Additional measurements with different catalyst particle sizes including industrial extrudates could be successfully described with the aid of a reactor model which took film and pore diffusion phenomena into account. (C) 2014 Elsevier B.V. All rights reserved.