2,4-Dihydroxybenzoic acid (2,4-DHBA) is found frequently as a pollutant in natural waters and represents a threat to water quality because it is a precursor to the formation of quinones which are highly toxic. The degradation of 2,4-DHBA using the vacuum UV photolysis of water has been investigated. Irradiation was carried out in an annular photoreactor equipped with a Xe-excimer lamp situated in the centre and emitting at 172 nm. The degradation kinetic followed a pseudo first order and the reaction has been found to be very heterogeneous, especially at low concentration. Impacts of oxygen or temperature have also been investigated but no effect has been shown. LC-MS and HPLC-UV combined with other analytical techniques allowed the identification of the formation of trihydroxybenzok acids and trihydroxybenzenes which underwent a ring opening, conducting to the formation of aliphatic products named alpha, beta, delta and gamma. These products were in turn degraded successively into malek acid, malic and succinic acid, malonic acid, glyoxalic acid and oxalic acid before reaching the complete mineralization in about 180 min. The proposed reaction pathway has shown to be very different from the one observed for the TiO2 photocatalysis which involves only holes (h(+)) without any formation of aromatic intermediates. The different behaviours of 2,4-DHBA towards the h(+) and HO+ make it a good probe to identify involved entities. (c) 2012 Elsevier B.V. All rights reserved.