Oxy-fuel combustion is usually performed in pf reactors under an enriched O2 atmosphere of CO2 to obtain a high CO2 content in the flue gases. The effect of the differences in thermal properties of N2 (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 °C under N2 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 N2. 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 N2-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. This work was carried out with financial support from the Principado de Asturias (PCTI 2013-2017, GRUPIN14-079), co-financed by the European Regional Development Fund (ERDF). Financial support from the CSIC (Project PIE 201380E064) is also gratefully acknowledged Peer reviewed