An increasingly important step in the design of three-way catalytic converters is having the ability to evaluate deactivation of potential catalyst formulations with accelerated testing. Most accelerated tests involve deactivating the catalyst by thermal (i.e., high temperature) and/or poisoning mechanisms using an engine/dynamometer. Efficient utilization of the engine/dynamometer tests involves optimizing the test conditions such that the largest amount of information can be gathered under the simplest and shortest test conditions. Optimization of the test, however, requires first the task of determining the mechanisms of catalyst deactivation during the test. Mechanisms of catalyst deactivation were studied for a (gamma-alumina) bead supported Pt/Rh three-way catalyst during the course of a high temperature accelerated test. High temperature aging experiments in the laboratory were coupled with the results of the accelerated test to gain in sight into the mechanisms causing the high temperature deactivation. Results of the laboratory aging experiments showed that loss of three-way conversion efficiency occurred readily under oxidizing conditions but not under neutral or reducing conditions. The loss of three-way conversion efficiency after exposure to high temperature oxidizing conditions could be regenerated by exposing the catalyst to high temperature reducing conditions. Oxidation of Rh was found to be the predominant deactivation mechanism in oxidizing conditions. Reduction of Rh oxide back to Rh metal explained the observed regeneration results. Regeneration of a catalyst exposed to the high temperature accelerated test also showed the mechanism of deactivation during the accelerated test was predominately one of oxidation of Rh.