Friction welding is a new technology in the course of which wood pieces are subjected to circular or linear movements in a welding machine. The wood in the contact zone is heated and melted. Aiming at a better understanding of the thermal alteration of wood and the formation of a viscous layer acting as adhesive, the thermally changed material was taken to be analysed by chemical and spectroscopic methods. As a manifestation of thermally splitting of wood, the total amount of extractable compounds within the joint material (JM) is higher than that of unaltered spruce. Cellulose was found less altered than the other essential wood components. Polyoses, on the other hand, are less stable under the conditions of friction welding; they are detectable only in small amounts in the altered zone. Lignin also suffers distinct changes as demonstrated by the increase of free phenolic groups and the decrease of the typical bonds between the phenolpropane units. Furan derivatives were detected within the volatiles of the smoke gas, arising mainly from the polyoses. It is obvious that reactions between furfural and other furan derivatives with lignin belong to the main reactions in the friction zone leading to cross-linking of the JM.