Catalyst development work was undertaken at W. R. Grace with the objective of designing an aftermarket converter having 20,000-25,000 mile durability, with a target price to the consumer of §100.00. Oxidation catalyst DC-1 and three-way catalyst DTW-1 with noble metal cost below corresponding OE catalyst were prepared and tested in the laboratory after pulsator aging to simulate 25,000 miles. Operation of these catalysts in an aftermarket converter with a design (frontal area, bed depth) different from OE converter was simulated with a mathematical model. The validity of the model for various air to fuel ratios was tested with experimental data. Model calculations and lab data indicate that Grace catalysts DC-1 and DTW-1, after 25,000-mile equivalent pulsator aging, will approximate the performance of 50,000-mile equivalent pulsator aged OE oxidation and three-way catalysts respectively.