The optimized design of an exhaust emission system in terms of performance, cost, packaging, and engine control strategy will be a key part of competitively meeting future more stringent emission standards. Extensive use of vehicle experiments to evaluate design system tradeoffs is far too time consuming and expensive. Imperative to successfully meeting the challenges of future emission regulations and cost constraints is the development of an exhaust system simulation model which offers the ability to sort through major design alternatives quickly while assisting in the interpretation of experimental data.Previously, detailed catalyst models have been developed which require the specification of intricate kinetic mechanisms to determine overall catalyst performance. While yielding extremely valuable results, these models use complex numerical algorithms to solve multiple partial differential equations which are time consuming and occasionally numerically unstable. Furthermore, keeping these kinetic data updated with improvements in catalyst formulations is also a very time consuming process. To resolve these issues, a simple model (SIMTWC, which stands for SIMple Three Way Catalyst) to predict tailpipe exhaust emissions has been developed by combining fundamental conservation expressions for mass and energy with a first order oxygen storage model and an empirical data base of steady-state catalyst performance obtained during engine dynamometer assessments, thus eliminating the need to specify the detailed kinetic mechanisms. Advantages of SIMTWC over more complicated techniques are computational expedience and robustness with comparable accuracy. Updates to SIMTWC as catalyst formulations change are also accomplished in a timelier manner.This paper describes the mathematical formulation of SIMTWC, solution technique, input data requirements, and provides a comparison of model results with vehicle experiments.