The photodecomposition of Inezin (S-benzyl O-ethyl phenylphosphonothiolate) in n-hexane under the ultraviiolet irradiation was investigated by gas liquid chromatography, thin layer chromatography, mass spectrometry, and infrared spectroscopy. Two main steps of photodecomposition were observed at the initial stage of irradiation. One was the cleavage of P-S bond to produce O-ethyl phenylphosphinate and dibenzyl disulfide, the latter of which was degraded rather rapidly to produce sulfuric acid and benzoic acid through toluene-alpha-sulfonic acid by oxidation. The other step was the isomerization to the thionate, O-benzyl O-ethyl phenylphosphonothionate, which was gradually oxidized to itx oxygen analogue, O-benzyl O-ethyl phenylphosphate. O-ethyl phenylphosphinate, O-benzyl O-ethyl phenylphosphonate and O-benzyl O-ethyl phenylphosphonthionate were fairly stable under the ultraviolet light, as compared with Inezin. As hydrolysis products of the parent compound, phenylphosphonic acid, O-ethyl hydrogen phenylphosphonate, O-ethyl hydrogen phenylphosphonothioate and benzyl alcohol were detected.