Char produced from the oil shale retorting process represents a significant environmental hazard and causes the waste of the resource due to its residual contents of aromatics and unconverted carbon. As an important step of oil shale retorting technology, the combustion of char can make full use of its heat value and the ash generated could be used as a solid heat carrier to favour oil shale pyrolysis. The reaction kinetics of char combustion was studied using a thermal gravimetric analyzer (TGA) under minimized internal and external diffusion conditions by applying the isoconversional method It was found that the average activation energy (E-a) of ex-situ char combustion was about 40.84 kJ/mol in the temperature range of 750 870 degrees C. The experimental char combustion data was fitted with different reaction models for its reaction mechanisms. A 3D nucleation and nuclei growth model had a good fitting coefficient of R-2 > 0.99 due to its assumed similarity to that of the real char combustion process. Combustion of char using different oxygen concentrations was investigated. The process E-a increased by about 20% when oxygen concentration was increased from 5 to 10%. Finally, combustion of in-situ char, i.e. hot char directly from pyrolysis, was compared with that of ex-situ (cold) char. The results showed that the E-a of in-situ char combustion was significantly reduced, by 32%, being even lower than the E-a of ex-situ char combustion using 20% oxygen.