The effects of ultrasonication on the humic acids extracted from the lignocellulose decomposed by anaerobic digestion were investigated via ultraviolet (UV) absorbency, Fourier transform infrared (FT-IR), H-1 nuclear magnetic resonance (H-1 NMR) spectra, gel permeation chromatography (GPC) and scanning electron microscope (SEM). The chemical modifications induced by ultrasonication mainly occurred in the carboxylic acid groups, the alpha-methylene groups on the aromatic rings, a variety of aliphatic carbons (mainly polymethylene), beta-aliphatic carbon on the aromatic rings and alpha-carbon oxygen bond. The SEM micrographs showed that the fibrous structures of the humic acids tended to aggregate after ultrasonication. An increase in the average molecular weight (MW) of the humic acids after ultrasonication was also observed in the GPC analysis. These results indicate that the ultrasound could not decompose the humic acid substrates completely. However, the results suggest the potential use of ultrasound as a basis for pretreatment before use of other methods to dispose of humic acids in contaminated water. (C) 2003 Elsevier B.V. All rights reserved.