The thermodynamic properties of phenoxathiin 10-oxide (PTO) and 135 polychlorinated phenoxathiin 10-oxides (PCPTO) in the ideal gas state at 298.15 K and 1.013×10⁵ Pa have been calculated at the BHandHLYP/6-311G** level using Gaussian 03 program. The isodesmic reactions were designed to calculate standard enthalpy of formation (Δ_fH⁽) and standard free energy of formation (Δ_fG⁽) of PCPTO congeners. The relations of these thermodynamic parameters with the number and position of Cl atom substitution (N_PCS) were discussed, and it was found that there exist high correlation between the thermodynamic parameters [entropy (S⁽), Δ_fH⁽ and Δ_fG⁽] and N_PCS (R²≥0.991). According to the relative magnitude of their Δ_fG⁽, the relative stability order of PCPTO congeners was theoretically proposed. Moreover, using a statistical thermodynamics calculation program based on Gaussian 03 output files, the values of molar heat capacity at constant pressure (C_p,m) from 200 to 1000 K for PCPTO congeners were calculated, and the temperature dependence relation of them was obtained, which suggests that there are very good relationships between C_p,m and temperature (T, T^－1 and T^－2) for almost all PCPTO congeners (R²＝1.000).

Key words: polychlorinated phenoxathiin 10-oxide, density functional theory (DFT), method of position of Cl atom substitution, thermodynamic property, relative stability