Removal of NOx from lean diesel exhaust can be achieved by the use of selective catalytic reduction technology. The supplied reductant is often ammonia, either as urea or as ammonia gas released from a storage medium. Experiments have been carried out on an engine test rig run to steady state conditions using NOx composed mainly of NO, with ammonia gas as the reductant. This was essentially a 1D study because a long 10 degree diffuser was used to provide uniform temperature and velocity profile to the SCR catalyst brick in the test exhaust system. Tuning of the standard reaction, the NO SCR reaction, in a kinetic scheme from the literature and adjustment of the ammonia adsorption kinetics achieved improved agreement between the measurements and CFD simulations. This was carried out for studies at exhaust gas temperatures between 200 and 300°C. The effect of diffuser geometry upstream of the SCR catalyst on NOx conversion was then investigated experimentally using a 180 degree sudden expansion as a 3D diffuser. These were also steady state studies with the exhaust NOx composed mostly of NO. The SCR brick was short, 45 mm in length, to provide a rigorous test of the kinetics. Observed NOx conversion profiles for ammonia supplied in quantities ranging from deficient to excess showed that the combined influence of temperature and velocity profiles upstream of the SCR was apparent in this 3D case. 2D axially symmetric CFD simulations have been carried out to model the 3D case and the predictions are discussed and compared with engine test data in this paper.