A novel scheme for simulating the selective catalytic reduction of NO with NH3 on a honeycomb-type catalyst was developed based on a 3-D model. New insight was gained by considering such modeling aspects as: correct geometry (square ducts); hydrodynamic entrance effects; occurrence of adsorption phenomena and side reactions; NH3/NO ratio in the feed; influence of the reactant (NO, NH3, O-2, H2O) concentrations; intraphase diffusion and interphase mass-transfer processes; and a large range of operating conditions (temperature 140-475 degrees C and gas hourly space velocity 14,800-73,900 h(-1)). These aspects proved to be crucial for interpreting and quantifying the experimental results and optimizing the catalytic reactor processes. The 3-D model was validated by comparing manufacturer＇s set point and experimental data to calculated data, and the external mass-transfer coefficients derived from the concentration gradients to the mass-transfer correlations available in the literature.