The effect of CO2 introduction in a biomass fast pyrolysis process at 850 degrees C was investigated. It was found that CO2 impacts the final gas yield and composition, and the char yield and properties. Introducing CO2 in the pyrolysis medium alongside nitrogen enhanced CO production as a result of homogeneous and heterogeneous reactions of CO2 with gases, tars and char. The char yield was lower compared to a reference char yield in pure nitrogen. The char obtained in a CO2-containing atmosphere has its surface area increased nearly sixfold and has a chemical composition different from that of chars obtained in N-2 atmosphere. However, the reactivity of the two chars towards H2O, CO2 and O-2 was almost the same. Temperature-programmed oxidation experiments on both chars - those obtained in pure nitrogen and those obtained in a CO2-containing atmosphere - revealed quite different oxidation profiles and peak temperatures. Taken together, these results tend to confirm that CO2 is impacting the biomass fast pyrolysis process. In the light of these results and of the literature findings, we propose a mechanism illustrating the role of CO2 during fast pyrolysis of biomass. (C) 2013 Elsevier Ltd. All rights reserved.