Accelerated aging of automotive catalysts has become a routine process for the development of new catalytic formulations and for homologation of vehicle emissions. In the standard approach, catalyst samples are subjected to very high temperatures of over 800°C on a pre-defined test cycle and aged for precise timescales representative of certain vehicle mileage. The high temperature feed gas is usually provided by a large gasoline engine but, increasingly, alternative bench-aging techniques are being applied as these offer more precise control and considerable cost savings, Offering more development possibilities. In the past few years, emissions control of diesel-engine vehicles has become prominent as more-stringent emissions legislation require complex after-treatment systems. Aging of the catalysts in these systems are not well understood as they are at subjected to a lower temperature environment than in a gasoline engine but at much higher oxygen levels. It is recognised that oxygen concentration has a significant influence on rate of aging but the current aging algorithms do not fully account for the effect of gas concentrations. In the research presented in this paper, the effect of oxygen concentration on the rate of catalyst aging is investigated and analysed. Several catalyst samples were aged over a precise temperature cycle for various aging times and at differing oxygen concentrations. The conversion performance of these samples was measured at regular intervals throughout the aging process to assess the stage and rate of thermal aging. The results were analysed in detail and compared with predictions based on the standard aging algorithm and with others proposed in literature.