It is well known that conversion as a function of temperature hysteresis can occur during ignition and extinction exothermic reaction experiments, such as CO oxidation over Pt/Al2O3, with the activity during the ignition process not matching that during the extinction process. Conversions being higher during extinction than that during ignition are often observed. Several explanations have been proposed in which heat effects, different catalyst surface states, and different Pt oxidation states are the most common. In this work CO oxidation hysteresis behavior, when in a mixture with C3H6, was investigated. The results show that when C3H6 was absent, CO oxidation followed normal hysteresis behavior; however, when C3H6 was added to the mixture, the catalytic activity during the extinction phase decreased. As the C3H6 concentration in the mixture increased, the hysteresis loop became smaller and ultimately reverse hysteresis was observed. The decrease in catalytic activity during extinction was due to the formation of C3H6 oxidation intermediate species. These species competed with CO for active sites, thus inhibiting CO oxidation, and were not present during ignition as CO was the dominant adsorbed species when starting at low temperature.