Thermodynamic properties, including entropy (S-Theta), heat capacity at constant volume (C-v(Theta)), enthalpy (H-Theta), and Gibbs free energy (G(Theta)) for 135 polychlorinated phenoxathiins (PCPTs) in the ideal gas state at 298.15 K and 101.325 kPa, have been computed using density functional theory (DFT) at the B3LYP/6-31G* level with the Gaussian 98 program. The standard enthalpy of formation (Delta H-f(Theta)) and the standard Gibbs free energy of formation (Delta(f)G(Theta)) were obtained by designing an isodesmic reaction. The relations of S-Theta, C-v(Theta), Delta H-f(Theta), and Delta(f)G(Theta) with the number and position of chlorine substitution (N-PCS) are discussed. It is suggested that S-Theta, C-v(Theta), Delta H-f(Theta), and Delta(f)G(Theta) of PCPTs vary greatly with the number and position of chlorine substitution. The values of heat capacity at constant pressure (C-p(Theta)) at temperatures (200 to 1800) K for PCPT congeners were calculated using a statistical thermodynamics calculation program based on Gaussian output files, and the temperature dependence relation of them was obtained using the least-squares method. On the basis of the magnitude of the relative standard Gibbs free energy of formation (Delta(R,f)G(Theta)), the relative stability of PCPT isomers was theoretically proposed in this work and then compared with that of polychlorinated dibenzothiophene (PCDT) isomers.