Degradation of humic acids extracted from seabed sediments, that were collected from two different locations in Japan, was carried out using an ozone treatment technique. Degradation was evaluated by UV254 and VIS400 absorbance, high performance size exclusion chromatography (HPSEC) and total organic carbon (TOC). Sediment was collected from Ago Bay, Mie prefecture and Ariake Bay, Fukuoka prefecture, Japan. After the ozone treatment, a decrease in absorbance of humic acids was observed over a wide range of pH. The UV254 absorbance of all humic acids showed a significant decrease. Furthermore, under neutral pH, more than 80% degradation was achieved for both sediments, with an initial humic acid concentration of 30 mg L-1. The total organic carbon (TOC) content of ozone-dosed solutions showed no significant changes. The decrease in the molecular weight of humic acids was evaluated using the values obtained by high performance size exclusion chromatographic (HPSEC) analysis. Humic acids in extracted solutions were related to the key materials in the sediment, which result from the semi-closed sea environment. Significant environmental improvement can be expected from the use of this technology. (C) 2008 Elsevier B.V. All rights reserved.