Thermodynamic parameters, including total electronic energy (E-t), enthalpy (H-circle minus), entropy (S-circle minus), Gibbs free energy (G(circle minus)), and standard heat capacity at constant volume (C-v(circle minus)) for 75 polychlorinated thianthrenes (PCTAs) in the ideal gas state at 298.15 K and 101.325 kPa, have been computed using density functional theory at the B3LYP/6-31G* level with the Gaussian 98 program. The relations of these thermodynamic parameters with the number and position of chlorine substitution (N-PCS) were discussed. It was suggested that E-t, S-circle minus, H-circle minus, G(circle minus), and C-v(circle minus) of PCTAs vary greatly with the number and position of chlorine substitution. The values of molar heat capacity at constant pressure (C-p,C-m) at temperatures from (200 to 1800) K for PCTA 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. The standard enthalpy of formation (Delta H-f(circle minus)) and the standard Gibbs energy of formation (Delta(f)G(circle minus)) were obtained by designing an isodesmic reaction. On the basis of the magnitude of the relative standard Gibbs energy of formation (Delta(R),(f)G(circle minus)), the relative stability of PCTA isomers was theoretically proposed in this work and then compared with that of polychlorinated dibenzo-p-dioxin (PCDD) isomers.