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Two major deactivation mechanisms of automotive catalysts during road usage are: 1. thermal aging 2. poison accumulation of oil-derived poisons such as zinc and phosphorus. A dynamometer-based aging cycle, incorporating a high-temperature low-poison mode to account for thermal aging followed by a low-temperature high-poison mode to account for poison accumulation, has been developed to allow the examination of catalyst formulations after exposure to both a harsh thermal and chemical aging environment. This type of aging cycle results in dynamometer-aged catalysts that are physically much more similar to road-aged catalysts than thermally-based dynamometer cycles. Using this aging-cycle, Pd-only, Pd-Rh and Pt-Rh light-off catalysts were examined. The Pd-Rh catalyst gave the best overall performance, with equivalent HC light-off performance to the Pd-only catalyst and equivalent NOx performance to the Pt-Rh catalyst.