Steady-state activity of two representative lean-NOx catalysts - Pt-ZSM-5 and Cu-ZSM-5 -has been analyzed for the NO + C2H4 + O-2 reaction under highly oxidizing conditions, typical of lean-burn gasoline engine exhaust. Results indicate that the steady-state activity of these catalysts exhibits a first-order homogeneous response with respect to changes in reactant feed concentrations. Identification of this unique kinetic behavior has enabled us to develop an activity normalization scheme by which the activity of a lean-NOx catalyst measured under different experimental conditions can be compared on an equal basis, provided the feed concentration ratios are kept constant. Also discussed is how the normalization scheme can be used for the efficient experimental design and the optimum control of lean-NOx catalysts for maximum catalytic activity along with its limitations.