Interesting chemical differences betwen weldlines obtained at 100 Hz and 150 Hz frequencies in linear vibration wood welding were observed by FT-IR and (13)C NMR. An increase in the proportion of unoxidized phenolic groups in the lignin is observed by FT-IR and the increased joint strength observed is ascribed to the improved adhesion derived from this. The much decreased level of oxidation due to the much shorter welding time possible at 150 Hz was clearly observed by (13)C NMR by a marked decrease, even disappearance, of the presence of dark coloured quinones in the weldline at 150 Hz. This yielded colourless weldlines. (C) Koninklijke Brill NV, Leiden, 2009