Mineralization of radioactive synthetic lignin (C-14-DHP) was studied in a compost environment at 35, 50 and 58 degreesC. Compost samples were successively extracted with water, dioxane and alkali, and the molecular weight distribution of some extracts was determined by gel permeation chromatography (GPC). Biodegradation of lignin-containing spruce groundwood (SGW) and pine sawdust was concurrently determined in controlled composting tests by measuring evolved CO2. The temperatures were the same as in the C-14- DHP mineralization experiment and bleached kraft paper, with a lignin content of 0.2%. was used as a reference. The mineralization of C-14-DHP was relatively high (23-24%) at 35 degreesC and 50 degreesC, although the mixed population of compost obviously lacks the most effective lignin degraders. At 58 degreesC the mineralization of 14C-DHP, as well as the biodegradation of SGW and sawdust, was very low, indicating that the lignin-degrading organisms of compost were inactivated at this temperature. SGW was poorly biodegradable (< 40%) in controlled composting tests compared with kraft paper (77-86%) at all temperatures, which means that lignin inhibits the degradation of carbohydrates. During the incubation. water-soluble degradation products, mainly monomers and dimers, and the original 14C-DHP were either mineralized or bound to humic substances. A substantial fraction of C-14-DHP was incorporated into humin or other insolubles.