Heat treatment helps enhance some properties of raw biomass by improving its decay resistance, its dimensional stability, increasing energy density and reducing transport costs of biomass. During storage period, many industrial sites undergo fires caused by self-ignition of torrefied or carbonized biomass. The main objective of this work was to study the chemical behavior of heat treated wood under oxygen exposure. Softwood and hardwood species have been thermally treated under a nitrogen atmosphere at different treatment conditions intensities. Sample mass and heat flow have been measured during the process to observe the temperature, time and air flow influence on reaction mechanisms of heat treated wood. The oxidation process and heat flux have been evaluated in addition. Results showed that reaction heat flows used for the treatment were correlated with temperature and time of thermal degradation of both examined wood species, as well as wood mass loss, respectively. However, hardwood (beech) seems to be more sensitive to thermal degradation and oxidation than softwood (silver fir) species. In addition, differential scanning calorimetry exothermic peak and wood mass gain were observed during oxygen exposure. In fact, this phenomenon was more pronounced for degradation carried out at high temperatures and times and it tends to be correlated with the elemental composition of wood. The main evolved products of heat treated wood were identified as water (H2O), carbon monoxide (CO) and carbon dioxide (CO2).