X-ray tomographic microscopy of wood fusion welded joints, without any adhesive, indicated that, in limited places, a reduction of intercellular material flow can occur in the bondline, and some breaks at the interface-fused composite/wood substrate can occur also. When proceeding from the outer faces to the innermost layers of the bondline, one passes first undamaged wood cells, followed by interphases in which cells are mixed with long wood cells, and where wood cells are far predominating over the fused matrix, to finally reach an innermost zone where the composite occurs and where the proportion of the fused matrix appears to predominate in relation to the fibers. The use of some naturally derived additives such as tannins and furfural, by autcondensation and polymerization, afforded some improvements in cold water resistance of the welded bondline. Electron Spin Resonance spectroscopy showed an increase in radical concentration in friction-sanded wood surfaces, indicating that some of the reactions occurring in the holding phase after wood welding are likely to follow a radical reaction route.