The effects of vehicle cold start operation and ambient air soak temperature on peak catalyst substrate temperatures were investigated by the United States Environmental Protection Agency (USEPA). Temperatures that could cause irreversible thermal deactivation of the catalyst were of concern. Internal catalyst substrate temperatures were monitored on twenty-two 1983 and 1984 model year high mileage properly maintained in-use vehicles during operation over the Federal Test Procedure (FTP) driving cycle. Vehicles were tested after nominal 20°F and 75°F ambient air temperature soaks.During cold start operation, catalysts of carbureted test vehicles with pump or pulse secondary air systems frequently reached peak temperatures where irreversible thermal deactivation could occur. During cold start operation, fuel injected test vehicles with pulse or no secondary air injection tended to exhibit, catalyst temperatures below the range normally associated with irreversible thermal deactivation. Cold (20°F) soaking often resulted in increased catalyst temperatures during cold start vehicle operation for some carbureted vehicles tested.