At present, 50% (w/w) carbon is widely promulgated as a generic value for wood; however, the literature yields few data and indicates that very little research has actually been done. C contents in heartwood of forty-one softwood and hardwood species were determined. C in kiln-dried hardwood species ranged from 46.27% to 49.97% (w/w), in conifers from 47.21% to 55.2%. The higher C in conifers agrees with their higher lignin content (similar to 30%, versus similar to 20% for hardwoods). Wood-meal samples drilled from discrete early wood and late wood zones of seven of the forty-one species were also investigated. C contents of early woods were invariably higher than those in corresponding late woods, again in agreement with early wood having higher lignin content. Further investigation was made into freshly harvested wood of some species to determine how much volatile C is present, comparing oven-dried wood meal with wood meal dried at ambient temperature over a desiccant. C contents of oven-dried woods were significantly lower, indicating that all past data on C content in oven- or kiln-dried woods may be inaccurate in relation to the true C content of forests. We conclude that C content varies substantially among species as well as within individual trees. Clearly, a 50% generic value is an oversimplification of limited application in relation to global warming and the concept of "carbon credits". (C) 2003 Elsevier Ltd. All rights reserved.