Surface free energies of pine and beech wood were investigated before and after heat treatment using the Lifshitz–van der Waals/acid–base approach from contact angles measured by the Wilhelmy method. The results obtained showed that the decrease of the electron-donating component of the acid–base component was the major parameter affecting the wetting of the modified wood's surface. The Lifshitz–van der Waals component was slightly modified after heat treatment indicating that the atomic and molecular interactions due to permanent or induced dipoles between wood macromolecules were weakly modified. Modification of the surface chemical composition was studied by X-ray photoelectron spectroscopy (XPS) and titration of acidity. XPS indicated an important decrease of the O/C ratio after heat treatment explaining the decrease of the electron-donating component (γ−) of the surface free energy. The decarboxylation and degradation of glucuronic acids present in hemicelluloses, demonstrated by titration of carboxylic acid functions of wood, had only limited effect on the electron-accepting component (γ+).