Vibration welding of wood that has been preheated according to an industrial two-step process indicates that such wood can be welded and can yield welded joints of good strength. The joint strength is, however, markedly lower than obtained when welding non-heat-treated timber. In general, weld strength of the timber is poor if welding is done on hydrothermolyzed wood. The strength results are instead much better if welding is done at the end of the complete heat treatment process, i.e., after the dry heat step. The weld lines of heat-treated wood show entangled cells where there is none or very little of the molten matrix intercellular material usually observed in welded timber. Furthermore, in weldlines obtained after hydrothermolysis an increase in rigidity and brittleness of the wood cells is observed. Hence, the wood cells are not entangled at all or very little. Both observations indicate that heat treatment has affected the main melting region of the wood, namely the intercellular material. As most of this material is already either lost or heavily cross-linked during heat treatment, only little of it is now available to melt and bind the wood surfaces during vibrational wood welding.