Di-n-butylphthalate, (DNBP) a ubiquitous environmental pollutant, has been decomposed under hydrothermal conditions between 300 and 380 °C in a batch reactor for 1 h. The reactions were carried out to monitor the effects of reaction temperature, oxidant concentration, and concentration of DNBP on the decomposition mechanism and product distribution. Generally, the oxidation of DNBP and its decomposition products increased with increasing temperature. At zero or lower hydrogen peroxide concentration, DNBP was hydrolyzed by water-catalyst to produce mainly residual benzoic acid which was formed from o-phthalic acid, a labile intermediate product. Other compounds found included phthalic anhydride, butylbenzoate, acetylbenzoate, 1-butanol, and 2-butanol. Effectively, the decomposition/oxidation of DNBP was limited by the resistance of benzoic acid to degradation. The same trend was noticed when the concentration of DNBP was varied; a higher concentration of the compound produced higher initial concentration of benzoic acid and less carbon dioxide. For example, only 66.8 wt % of the carbon content of 105.0 mg L-1 of DNBP was obtained as carbon dioxide compared to 94.5 wt % when 10.5 mg L-1 was reacted using 6 wt % of oxidant.