In order to further characterize and optimize the performance of Selective Catalytic Reduction (SCR) aftertreatment systems used on heavy-duty diesel engines, an accurately calibrated high-fidelity multi-step global kinetic SCR model and a reduced order estimator for on-board diagnostic (OBD) and control are desirable. In this study, a Cu-zeolite SCR catalyst from a 2010 Cummins ISB engine was experimentally studied in a flow reactor using carefully designed protocols. A 2-site SCR model describing mass transfer and the SCR chemical reaction mechanisms is described in the paper. The model was calibrated to the reactor test data sets collected under temperatures from 200 to 425 °C and SCR space velocities of 60000, 90000, and 120000 hr-1. The model parameters were calibrated using an optimization code to minimize the error between measured and simulated NO, NO₂, N₂O, and NH₃ gas concentration time histories. The calibrated SCR model is able to predict the effect of temperature and flow rate on SCR kinetics and is capable of predicting gaseous NO, NO₂, NH₃, N₂O, and NH₃ storage on the catalyst. The SCR model and the protocol used for the reactor tests are described along with the experimental data as compared to the model results for NO, NO₂N₂O and NH₃ out of the SCR catalyst.