Abstract: The 35 degrees and 50 degrees C isotherms of juvenile Pinus sylvestris L. wood from recently cut trees were compared with those of juvenile wood of the same species previously forming part of an 18(th) century wooden building in order to determine the thermodynamic properties of the two types of wood through the isotherms. The isotherms were plotted using the gravimetric method of saturated salts in the water activity range of 0.11 to 0.97 for the 35 degrees C isotherm and 0.11 to 0.96 for the 50 degrees C isotherm. The sorption curves were fitted using the GAB method, and the isosteric heat of sorption was obtained by means of the integration method of the Clausius-Clapeyron equation. In both types of wood, the net isosteric heat decreases as the moisture content of the specimen increases, and the maximum values of isosteric heat in the new wood are greater than in the old wood, both in adsorption and desorption. This indicates that the bond energy in the new wood is greater than in the old wood.