MnCr2O4 and Cr2O3 are two important protective oxides on the surface of austenitic alloys used for ethylene cracking. This investigation is focused on the effect of CeO2 on the stability of MnCr2O4 and Cr2O3 in high temperature carbonaceous environments. The presence of 5% CeO2 in MnCr2O4 increased the carburization rate of the MnCr2O4 spinel through the formation of CeCrO3, which accelerates the decomposition of MnCr2O4. CeCrO3 was formed when 5% CeO2 was added to Cr2O3. Compared with Cr2O3, CeCrO3 appears more stable. However, CeCrO3 can be further decomposed to CeO2 and chromium carbide in high temperature carbonaceous environments. The latter quickly formed nanosized carbides, which induced coke formation during increased exposure in carbonaceous environments. In carbonaceous environments with higher oxygen partial pressures, the detrimental effect of CeO2 on the carburization resistance of the MnCr2O4 spinel and Cr2O3 is alleviated because of the suppression of CeCrO3 formation, thereby preventing coke formation. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3478659] All rights reserved.