The production of useful organic matter from sludge expands the potential for utilizing the product as a resource. This paper is focused on the accumulation and removal of volatile fatty acids (VFAs) produced from primary, secondary and a mixture of both sludges using hydrothermal treatment with and without the use of oxidants. The results confirmed that the maximum accumulation of VFAs was achieved at moderate hydrothermal treatment temperatures below 200 degrees C, and was dependent on the composition of sludge and the amount of oxidant used for treatment. The maximum accumulation reached 0.26-0.36 mg VFAs as COD per mg VS from the three sludge types. Acetic acid dominated the VFAs and was followed by propionic acid, with the other VFAs accounting for less than 5% of the total VFAs. As hydrothermal treatment progressed in terms of COD removal, the contribution of acetic acid to the total VFAs increased while the contributions of the other VFAs decreased. The VFAs constituted a major component of the accumulated dissolved organic matter, which reached a maximum of 0.5-0.7 mg soluble COD per mg VS from the three sludge types.