Abstract In a chemical looping combustion process, as the oxygen carrier circulates the system, it is subjected to morphological and compositional changes such as sintering, attrition and reaction between metal oxides and fuels. These changes may cause the deactivation of the oxygen carrier to decrease over time. This research work is carried out to investigate and characterise the iron oxide and copper oxide particles used in a chemical looping combustion system after 20 reduction–oxidation cycles with 5 different fuels, (i.e. Hambach lignite coal, Hambach lignite char, activated carbon, Taldinski bituminous coal and US bituminous coal). Oxygen carriers used in this study were subjected to analyses with X-ray diffraction (XRD), inductive coupled plasma mass spectrometry (ICP-MS), scanning electron microscope (SEM) and BET surface area and full isotherm analysis. XRD analysis on these particles showed that the reduced particles were reoxidised back to their original form with trace amounts of impurities such as silicon dioxide (SiO 2 ) on the iron oxide particles whereas similar impurities plus copper complexes such as copper aluminate were detected on the copper oxide particles. ICP-MS analysis on these particles revealed that the concentration of the core metal elements (iron, copper and alumina) decreased after 20 reaction cycles which in turn decreased the total concentration of the metal elements in the particles. BET surface area analysis on these particles indicated that the total surface area of these particles decreased by more than 50% of their original values. Full isotherm analysis on the copper oxide particles showed that the total pore volume of the particles decreased after 20 reaction cycles even though the pore sizes of the particles increased slightly. SEM analyses showed the iron oxide particles underwent severe attrition whereas the copper oxide particles showed signs of agglomeration due to the high operating temperature.