The elemental compositions and yields of CuO-derived phenol dimers and monomers from woods degraded by different fungi under laboratory and natural conditions were compared to those from undegraded controls. In laboratory experiments, white-rot fungi caused pronounced mass losses, lowered the organic carbon content of the remnant woods, and decreased the absolute carbon-normalized yields of the major classes of lignin phenol dimers and monomers. White-rot decay induced large losses of some CuO reaction products, such as (β,1-diketone and α,l-monoketone dimers and syringyl monomers, and increased the absolute yields of individual acidic reaction products, such as dehydrodivanillic acid, vanillic acid, and 2-syringylsyringic acid. In contrast, the brown-rot fungus, Fomitopsis pinicola, was less efficient in decaying lignin, inducing lower absolute lignin phenol losses and, in some cases, increasing the organic carbon content of remnant woods. Several lignin constituents, mainly carboxyvanillyl monomers and α,2-methyl and α,5-monoketone dimers, were produced during brown-rot degradation. Similar diagenetic trends were also apparent in the five woods collected from the field, suggesting the differences between white- and brown-rot decay are still apparent after more extensive degradation in natural environments. The lignin compositions from a selected set of previously analyzed sedimentary mixtures were generally consistent with the diagenetic trends observed in both laboratory and field samples. In some cases, however, geochemical parameters such as elevated dimer/monomer and carboxyvanillyl/ vanillyl monomer ratios clearly distinguished certain sedimentary lignins. In these samples, other processes, such as extensive fungal decay, bacterial degradation, or a nonwoody vascular plant origin, could be important factors affecting lignin compositions.