Beech wood has been treated by impregnation followed by heating at various temperatures with solutions containing citric acid (CA) or tartaric acid (TA) alone or in combination with glycerol (G), i.e. with G+CA and G+TA. The resulting modified woods were tested in terms of resist-ance to leaching, durability and dimensional stability. These properties are improved as a function of heating tem-perature, which leads to higher levels of poly-esterification involving grafting onto wood simultaneously with thermal degradation of wood. Dimensional stability of all treated wood was increased, but glycerol does not have a positive effect with this regard. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy contributed to understanding the effects of the different treatments. In situ polymerization of G+TA at 140°C increased the bending resistance, while G+CA polymerization does not compen-sate notably the mechanical weakness induced by thermal degradation of wood at higher temperatures. However, G+CA treatment is more efficient regarding leaching and decay resistance, than that with G+TA.