The determination of surface free energy of wood is an important parameter in applications such as coating, painting and gluing. The simplest technique for that is measurement of contact angle. This technique has been used for the investigation of the effect of heat treatment of wood on the surface free energy. Two wood species – aspen and grey alder – have been treated in a range of temperatures from 140 to 180°C. With the sessile drop technique, series of contact angle data have been collected and used to determine the corresponding surface energies and changes in wettability. The most commonly used approaches, Zisman, Owens–Wendt, Wu, equation of state and acid–base theories, were applied. The results demonstrate that heat treatment affects wettability and solid surface energy, in particular, from the Owens–Wendt and Wu theories. We know that at higher treatment temperatures, the polar component of the surface energy for both wood species decreases, while the dispersive component is almost constant. The experiments have been conducted while trying to fulfil the same measuring conditions, but in any case, it was not possible to reach the thermodynamic equilibrium conditions assumed by Young’s equation, due to chemical heterogeneity, surface roughness and capillarity. It is possible to improve the accuracy of results using a larger number of probe liquids.