The soil mold Paecilomyces inflatus is capable of modifying and partially mineralizing synthetic and natural humic acids (HAs) in compost environments. HA degradation studies using a synthetic HA (C-14-HA) in autoclaved compost microcosms showed that, after 12 weeks of cultivation, P. inflatus mineralized approximately 5% of the C-14-labeled HA to (CO2)-C-14, while 6% of the C-14-HA was converted into C-14-labeled water-soluble fragments (fulvic-acid-like fraction). About 40% was still present as NaOH-soluble HA representing unmodified or only slightly modified humic material ( compared with 60% in the controls). Modification of natural HAs extracted from compost was followed by their partial decolorization (30%) in liquid cultures of P. inflatus. Bleaching of the medium was accompanied by moderate changes in the molecular mass distribution of both the HA and fulvic-acid fractions, which were analyzed with high-performance size exclusion chromatography. HA modification was most pronounced during the primary growth phase of the fungus and was associated with increased laccase activity.