Standard enthalpy, standard Gibbs energy, standard entropies, and standard heat capacities at constant volume for 136 polychlorinated dibenzothiophenes (PCDTs) in the gaseous state at 298.15 K and 101.325 kPa have been computed using the density functional theory (B3LYP/6-31G*) with Gaussian 98. On the basis of the output data of Gaussian, isodesmic reactions were employed to calculate the standard enthalpy of formation and the standard Gibbs energy of formation of the 136 PCDTs in the gaseous state at 298.15 K and 101.325 kPa. The correlation equations between the standard entropy, standard heat capacity at constant volume, the standard enthalpy, the standard Gibbs energy, and the position of Cl substitution (N-PCS) were obtained, where all R-2 values are larger than 0.98. On the basis of the magnitude of the relative standard Gibbs energy of formation, the relative stability of PCDT isomers was theoretically proposed in this work and compared with that of polychlorinated dibenzofurans (PCDFs). In addition, the correlations between structural parameters and the position of Cl substitution (N-PCS) were discussed. The good correlations were found between molecular average polarizability, energy of the highest occupied molecular orbital, energy of the lowest unoccupied molecular orbital, molecular volume, and N-PCS, and all R-2 values are larger than 0.97.