The kinetics of the selective catalytic reduction of nitric oxide by ammonia have been investigated over a vanadia-based commercial DeNO(X)-catalyst. For temperatures exceeding 200 degrees C the kinetic data are described well with a model which is based on an Eley-Rideal mechanism. Below 200 degrees C the conversion is higher than predicted with this model, probably due to the contribution of an additional reaction pathway, which occurs only at low temperatures. Regarding the influence of water and oxygen on SCR, O-2 accelerated and H2O decreased the reaction rate. The addition of up to 2% H2O to the dry feed notably reduced the rate of NO conversion. The effect levels off for higher H;?O concentrations, and between 2.5% and 7.5% H2O no significant dependence was perceptible. Similarly, the conversion of NO is almost independent on O-2 for concentrations between 4% and 10%. Lower concentrations resulted in a marked decrease of the catalytic activity. The estimated activation energies for the dry and wet feed amounted to 87 kJ/mol +/- 3 M/mol and 99 kJ/mol +/- 9 kJ/mol, respectively (95% confidence limits).