A catalyst activity function for the commercial Pd-based three-way catalyst (TWC) has been developed to describe the effect of Pd loading ranging from 20 to 240 g/ft.(3) on its catalytic performance. Derived from the alteration of the Pd metallic surface area (MSA) due to the variation of the Pd loading, the activity function reasonably well captures the nonlinear dependence of catalytic activity on the Pd loading of the Pd-based TWCs. The overall reaction kinetics for the commercial TWC obtained from the combination of the catalyst activity function developed and the primary reaction kinetics over the 4k Pd240 reference catalyst has proven to be capable of well predicting the effect of Pd loading on the catalytic performance of the Pd-based TWCs. The accuracy of model predictions has been excellent for the oxidation of CO, C3H6 and H-2, while leaving some room for improvement in the model accuracy for the reduction of NO and related byproducts. (C) 2012 Elsevier B.V. All rights reserved.