Chemical modification of wood with bio-polyesters, namely polylactic acid (PLA) and polybutylene succinate (PBS), has recently been reported (Noël et al. 2015, Grosse et al. 2017a). Both oligomeric treatments (OBS and OLA) confer interesting properties to wood for outdoor applications. They are based on the impregnation of oligomers in oven-dried beech wood (Fagus sylvatica L.), followed by a curing step at 100 or 120°C for OBS or 160°C for OLA. On the contrary of OLA, the dry curing conditions did not allow any penetration of OBS in wood cell walls, thus any anti-swelling efficiency (ASE). Humid treatment conditions were observed to allow a partial diffusion of OBS in cell walls, most likely because of water acting as swelling agent. Hydro-thermal treatment (HTT) was carried out at several temperatures, relative humidity and duration, to find optimal conditions of OBS diffusion in the wood structure. Treatment conferred ASE of 60% to 70%, with a good correlation to OBS diffusion. (Grosse et al. 2017b). In this study, dynamic mechanical thermal analysis (DMTA) under varying humidity was carried on for understanding of OBS/OLA interactions with wood polymers. First, temperature scans were performed from -150°C to 150°C at a heating rate of 5 °C/min and for a frequency of 1, 10 and 100 Hz. Glass transitions of wood components and oligomers appeared in the tan  response of the modified wood. Then relative humidity in the chamber was set at 35% and the temperature changes were manually performed when the conditions in the chamber were stable. Wood impregnated with OBS but not hydrothermally-treated has the same behaviour as untreated wood indicating few interactions between the polymer and the cell walls. This is well correlated with the low diffusion of OBS into wood structure. In the case of hydrothermally-treated samples the same global behaviour can be observed. In the case of OLA treated wood, stiffness of the material decreases more rapidly with the temperature in comparison with the OBS treated wood. Oligomers of lactic acid, more hydrophilic and smaller than the PBS ones, are more likely diffusing then interacting with the cell walls polymers.