Chemical looping combustion (CLC) is a method for combustion with inherent CO(2) separation. In CLC an oxygen carrier supplies the oxygen necessary for fuel conversion. A method to determine the enthalpy of the reaction of oxygen carriers was developed. This method utilizes the fact that for many oxygen carriers the reaction enthalpy with CO is exothermic while it is endothermic with CH(4). By measuring the temperature change for reduction of different mixtures of CO and CH(4) it is possible to find the mixing ratio giving a thermally neutral reaction. From the known reaction enthalpies of CO and CH(4) with oxygen it is then possible to deduce the reaction enthalpy of the oxygen carrier. The reaction enthalpy for a NiO based oxygen carrier is estimated to 475 +/- 8 kJ (mol O(2))(-1). The expected value of 469 kJ (mol O(2))(-1) is in the error range and thus verified the method. The reaction enthalpies for three different ilmenite particles were also determined to 453 +/- 12 kJ (mol O(2))(-1) for synthetic ilmenite and 469 +/- 5 kJ (mol O(2))(-1) for natural ilmenite. A natural ilmenite that has previously been long term tested ended up with the reaction enthalpy 468 +/- 9 kJ (mol O(2))(-1). (C) 2011 Elsevier B.V. All rights reserved.