Organophosphorus esters (OPEs), a series of high production volume chemicals, receive increasing concerns due to their occurrence in worldwide aquatic environment and resistance to conventional biological water treatment processes. Degradation of tris(2-chloroethyl) phosphate (TCEP) was tested with 185 + 254 nm ultraviolet irradiation treatment. Transformation of TCEP followed a pseudo-first order kinetics with an apparent rate constant at 1.65 x 10(-3) s(-1) when [TCEP](0) = 3.51 mu M, 185 vacuum ultraviolet (VUV) irradiation intensity = 0.24 mW cm(-2). Degradation experiments with wavelength screening, radical scavenging and kinetic calculation revealed that the radical-based oxidation induced by 185 nm VUV was the dominant mechanism with a reaction rate constant k.(OH-TCEP) at (3.2 +/- 0.3) x 10(8) M-1 s(-1). Radical-based addition and substitution of TCEP generated intermediate products with phosphate backbone at early stage (0-20 min). Based on the toxicology analysis including reactive oxygen species and apoptosis of Escherichia coli, these early stage products have lower toxicity than TCEP or their further small molecule products. Variable pH value and natural organic matters/anions affected the efficiency of VUV, while intrinsic VUV screening and radicals scavenging were two key inhibition mechanisms. VUV induced a radical-based oxidation of TCEP, and it will be a promising treatment method for transforming and detoxifying micro-pollutants from water.