The molecular structure of humic acid (HA) extracted was investigated by FT-IR and C-13 CP/MAS NMR spectroscopy during the vermicomposting of sugar industry wastes, viz. pressmud, trash and bagasse for 60 days. A rapid decrease in UN and lignocellulosic (lignin, cellulose, hemicellulose) content was observed in vermicompost during early phase of the process. The FT-IR and C-13 CP/MAS NMR spectra of HA indicated a high rate of change in structure with increase in the alkyl C/O-alkyl C ratio during the process. Aromatic structures and carboxyl groups showed an initial increase but decreased after similar to 40 days indicating extensive mineralization during final stages of vermicomposting. (c) 2006 Elsevier Ltd. All rights reserved.