Use of selective catalytic reduction technology is the most popular strategy for removing NOx from lean diesel exhaust. The reductant is essentially ammonia and this has been supplied as a spray of urea droplets, but more recently alternative technology where ammonia gas is released from a storage medium has become a viable alternative. Experiments have been carried out on an engine test rig run to steady state conditions using NOx composed of either 25% or 50% of NO₂, with ammonia gas as the reductant. This was a 1D study where a long 10 degree diffuser provided uniform temperature and velocity profiles to the SCR catalyst brick. Under the transient conditions that occur during drive cycles, the dosing of the ammonia can deviate from the optimum. In this study, the dosage rate of ammonia was held at a fixed value, while the engine load was varied. The variation was from low load to high load and back down for various time periods, based on the rates of change that are typical of those in engine drive cycles. A study where the change was from low load to high load but then remained high was also performed, and also where the change was from high load to low. The low engine load supply temperature was about 215°C, rising towards 300°C under steady state conditions at high load. The NO and NO₂ levels downstream of the SCR were measured using fast response CLD NOx analyzers in some of the experiments. The latter are sensitive to cross talk from ammonia so ammonia was under-dosed in those experiments and there was negligible ammonia slip downstream of the SCR where the CLD measurements were taken. In other experiments an FTIR analyzer was used and higher ammonia dosing levels were investigated. The transients observed have been modeled in a CFD model using modified standard SCR reaction kinetics in a full kinetic scheme obtained from the literature. In the cases where the % of NO₂ was about 25 it was necessary to enhance the slow SCR reaction rate to simulate the observations, but in the cases where the % of NO₂ was about 50 the fast SCR reaction dominated and gave a reasonable description of the observations. The model's ability to simulate transients in an SCR engine exhaust system is assessed in this paper.