The photodegradation of humic acid (HA) was carried out in the presence of the Fenton reagent. The absorbance decrease of HA was strongly influenced by the pH, and initial concentrations of H2O2 and Fe(II). An initial absorbances of HA (10 mg L-1) in 254 and 400 nm were completely disappeared after 8 h under the optimum conditions. The decrease of TOC as a result of mineralization of HA was observed during the photo-Fenton process. The degree of HA mineralization was about 80% under UV irradiation after 15 h. The molecular weight distribution changes of HA were evaluated by high performance size exclusion chromatography (HPSEC). The large molecular weight component in HA appears to be easily degraded by the photo-Fenton process than the smaller components. Furthermore, the photo-Fenton process was successfully applied to the degradation of HA in sea sediment of Ago Bay, Mie Prefecture, Japan. Based on these results, the photo-Fenton reaction could be useful technology for the treatment of environmental sample contaminated by HA. (c) 2007 Elsevier B.V. All rights reserved.