Friction welding of wood at 220 Hz for a few seconds results in changed appearance of the material in the solid weld joint and in centimeter-scale outflow of material in a “flash.” Molecular-level structural changes in the weld of pine and maple woods and in maple-wood flash were investigated by quantitative solid-state 13C NMR, supplemented by 13C and 1H relaxation-based spectral editing. Care was taken to avoid mechanical decrystallization of cellulose in the sample preparation. Compositional changes in the absence of grinding artifacts were surprisingly subtle, given the changes in sample appearance and the high temperatures reported to occur during friction welding. The crystalline fraction of cellulose in the weld and flash remained nearly unchanged. Changes in NMR peak positions documented transformation of crystalline cellulose Iα to Iβ. Between 11 and 19% of cellulose surface chains and acetylated hemicellulose, which account for ca. 30% of the wood, were transformed chemically, for instance into aromatic, C-CH2-C, and C = O moieties. In pine weld, the observed increase in aromaticity was due to O-containing aromatic rings formed near cellulose and not lignin, as proved by 1H inversion recovery with 13C detection and consistent with the increase in aromatic C–O signal intensity. In maple weld, a similar observation was made, although the analysis was complicated because 1H relaxation of lignin was inhomogeneous. The flash material showed evidence of additional pyrolytic transformations. Implications for the behavior of wood like a thermoplastic material during welding are discussed.