The chemical changes occurring upon rotational welding with dowels of Canadian wood species sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) were examined by pyrolysis-GC/MS, DSC and XPS anlayses. The analyses performed separately on wood substrate (reference wood) and welded material by pyrolysis-GC/MS, DSC and XPS indicate that the differences in mechanical performances of the two welded woods are due mainly to the differences in original lignin structures as well as in the welding temperatures determined for the two wood species. The more pronounced guaiacyl character of the maple wood lignin seems to explain the preferential condensation reactions of the guaiacyl moieties in maple lignin with formaldehyde and furanic compounds released from lignin and carbohydrates during the fast pyrolysis associated with the welding process. The higher temperature determined for maple wood welding than for birch could be responsible for enhanced miscibility of wood polymers in the welding zone, explaining, therefore, the more significant presence of xylan polymer together with newly formed lignin carbohydrate complex (LCC) in the welded material. The detailed analysis of the compounds identified by pyrolysis-GC/MS, together with the results of the other two methods applied in this study, has confirmed that the S/G ratio cannot be taken as the sole criterion for the discussion of the chemical changes in lignins during welding of wood. (C) Koninklijke Brill NV, Leiden, 2010