In this study, naturally occurring ilmenite and manganese-modified ilmenite were evaluated as oxygen carriers with respect to their O-2 uncoupling behaviour. The effects of temperature and pre-treatment on the O-2 uncoupling properties of manganese-modified ilmenite were also investigated. The experimental results indicated that both calcined ilmenite and manganese-modified ilmenite were capable of releasing gas-phase O-2 at certain temperatures with the amount of O-2 released by the manganese-modified ilmenite being much more than that of the calcined ilmenite. When the manganese-modified ilmenite was pre-treated by subjecting it to several cycles of chemical looping combustion with CH4 as reductant, the Mn2O3 might react with iron species that had migrated outwardly, resulting in the formation of a ferrimanganic oxide which exhibits favourable thermodynamics for the oxidation reaction for the reduced oxygen carrier. As a result, the reduced pre-treated oxygen carrier could be easily re-oxidized and more O-2 could be released by the pre-treated oxygen carrier during every uncoupling steps than that by the untreated one. More gas-phase O-2 was released by the manganese-modified ilmenite when more redox cycles were performed during pre-treatment. The temperature of isothermal uncoupling-oxidation cycling showed a significant influence on the O-2 uncoupling behaviour for manganese modified ilmenite. When temperatures were either higher or lower than 900 degrees C, less O-2 was released from manganese-modified ilmenite. 30%-Mn2O3/ilmenite released more O-2 than 15%-Mn2O3/ilmenite when the temperature was below 950 degrees C while the same amount of O-2 was released when the temperature was 1000 degrees C.