A theoretical investigation of the atmospheric oxidation of a cyclic organosulfur compound 1,4-thioxane by O-3 is performed. The pathways for the reaction of 1,4-thioxane with O-3 have been modeled using B3LYP, M06-2X, MPW1K and MP2 level of theories with 6-31G(d,p), 6-311G(d,p) and 6-31+G(d,p) basis sets. The reaction is initiated by the formation of a primary ozonide, followed by a biradical, which on subsequent reactions with other atmospheric species produces hydroxyl radical, hydrogen peroxides and organic peroxides. The results obtained from DFT calculations were subsequently used to perform canonical variational transition-state theory calculations to determine the rate constant. The calculated rate constant is in good agreement with the available experimental data. (C) 2012 Elsevier B.V. All rights reserved.