Oxy-fuel combustion is usually performed in pf reactors under an enriched O-2 atmosphere of CO2 to obtain a high CO2 content in the flue gases. The effect of the differences in thermal properties of N-2 (conventional air combustion) and CO2 (oxy-fuel combustion) on the devolatilization process needs to be evaluated. The morphology and reactivity of biomass chars obtained by devolatilization in an EFR (entrained flow reactor) at 1300 degrees C under N-2 and CO2, simulating air and oxy-fuel combustion atmospheres, were studied. Four biomasses were selected: PIN (pine sawdust), OW (olive waste), OS (olive stones) and CW (coffee waste). The apparent volatile yield under CO2 was greater than under N-2. The morphology of the chars was assessed using SEM (scanning electron microscopy). The higher mass loss and the lower char particle size obtained during CO2 devolatilization indicate that a char-CO2 reaction occurred. The reactivity indices indicate a lower reactivity of the CO2-chars than the N-2-chars. The devolatilization atmosphere had a significant effect on the biomass chars, suggesting that gasification had occurred during CO2 devolatilization. The OW, OS and CW chars showed a very high reactivity up to intermediate conversion levels, probably due to the catalytic effect of inherent alkali metals. (C) 2015 Elsevier Ltd. All rights reserved.