Wood heat treatment is an attractive alternative to improve decay resistance of low natural durability wood species. Nowadays, several types of thermal treatments of wood exist. These treatments differ mainly by the nature of the inert atmosphere used to avoid wood combustion which may correspond to: nitrogen, vacuum, steam or oil. Decay resistance is strongly correlated to thermal degradation of wood cells wall components and consequently of treatment conditions from which depend chemical modifications. The aim of this study is to determine the effect of a new generation of treatment performed under vacuum on the chemical modifications occurring during treatment. For this purpose, one hardwood (beech) has been treated under different conditions (vacuum, nitrogen, steam). All treatments were performed at 220 °C for mass losses resulting from wood thermodegradation of approximately 12%. For each treatment condition extractives, Klason lignin, hemicelluloses and α-celluloses content were determined as well as monosaccharide composition. Intensity of thermodegradation was also evaluated by TD-GC-MS. Results show that extracts content were lower in the case of wood samples treated under vacuum, while lignin, hemicelluloses and α-celluloses contents were higher in the case of samples treated under steam and nitrogen, indicating lower wood degradation under vacuum treatment. These results were confirmed by chromatography analyses according to the nature and quantity of thermodegradation volatile products formed during TD-GC-MS analyses.