To aid in understanding black carbon (BC) formation during smoldering combustion in forest fires, we characterized charring of a softwood and hardwood. Charring (150, 340, 480°C) caused mass loss (7-84%), enrichment of organic carbon (OC) (0-32%), and 13C depletion (> 150°C). As determined by 13C MAS NMR, the OC composition of the woods was dominated by (di)-O-alkyl structures, and the chars by alkyl and aromatic structures. With increasing temperature, aromatic structures increased and the chars became more similar, although initial differences in OC concentration and δ 13C of woods persisted. The BC cluster sizes apparently remained small, pointing towards a low resistance against oxidation. Crown Copyright © 2002 Published by Elsevier Science Ltd. All rights reserved.