The TiO2 photocatalytic decomposition of ethyl-S-dimethylaminoethyl methylphosphonothiolate (VX) as a real chemical warfare agent (CWA) in liquid phase has been investigated by gas chromatography/mass spectrometry (GC/MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. The TiO2 photocatalytic decomposition rate, r(VX), was accelerated with increase in alkalinity. The several kinds of by-products such as diisopropylamine, ethyl methylphosphonic acid, oxygenated VX and partially oxidized VX were observed. On the basis of the by-products, the TiO2 photocatalysis were proposed as the P-S, C-N cleavage and oxygenation of VX molecule carried out mainly. The difference between r(VX) and vaporized dimethyl methylphosphonate (DMMP), r(DMMP) as CWA simulant was also investigated and the r(VX) was 30 times faster than the r(DMMP). On the basis of the decomposition mechanism of VX and DMMP molecules, the results of the acceleration of r(VX) in alkaline condition and of the comparison of r(VX) and r(DMMP) were also discussed. (c) 2013 Elsevier B.V. All rights reserved.