The steady-state kinetics of the selective catalytic reduction of NO with NH3 have been studied over a 2 wt% Mn/Al2O3 catalyst at 423 K using wide concentration ranges for NO, NH3 and O-2. Rate equations have been derived which are based on a previously elucidated mechanistic model including parallel Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) pathways to N-2 production and inhibition by product water. These equations provide significantly better fits of the kinetic data than rare equations based on either LH or ER mechanisms and may be used for further reactor design.