The semibatch wet air oxidation of aqueous solutions of linear alkylbenzene sulfonate (LAS), an anionic surfactant, has been investigated at temperatures of 453 and 473 K, total pressures of 2.8 and 3.3 MPa, and reaction times varying from 40 to 390 min The concentration of total organic carbon, chemical oxygen demand, and active detergent were followed throughout the reaction, the main intermediates formed were identified by means of electrospray-MS and highperformance liquid chromatography, and a mechanism describing LAS oxidation was proposed. It was found that LAS could easily be oxidized at 473 K to yield a group of molecules with short alkyl chains that do not behave as active detergents. The segments of alkyl chains broken off the reaction intermediates appear primarily as short-chain organic acids that are resistant to total oxidation. Oxidation experiments were also performed at 473 K with solutions of 4-hydroxybenzene sulfonic acid, an intermediate formed during the oxidation of LAS, It was found that 4-hydroxybenzene sulfonic acid could easily be oxidized.