We debarked, chipped, and ground a birch wood (Betula sp.) stem into sawdust (0.5-15 mm particle were size selected). Prior to the HTT, we subjected the sawdust to acetone (100%) extraction to remove the lipophilic extractives and eliminate their influence on the determination of lignin yield and structure. We purchased acetone (C 3 H 6 O, 95 vol %) and used it without purification, as well as deuterated dimethyl sulfoxide (DMSO-d6) for NMR spectroscopy. Both chemicals were of analytical grade and purchased from Sigma-Aldrich.