The thermodynamic properties of 135 polybrominated dibenzothiophenes (PBDTs) in the gaseous state at 298.15 K and 1.013 x 10(5) Pa, are calculated using the density functional theory (the B3LYP/6-311G**) with Gaussian 03. Based on these data, the isodesmic reactions are designed to calculate the standard enthalpy of formation (Delta H-f(0)) and the standard Gibbs energy of formation (Delta(f)G(0)) of PBDTs. The relations of these thermodynamic parameters with the number and position of bromine substituents (N-PBS) are discussed, and it is found that there exist good correlations between thermodynamic parameters (including heat capacity at constant volume, entropy, enthalpy, free energy, Delta H-f(0), Delta(f)G(0)) and N-PBS. T e relative stability order of PBDT congeners is proposed theoretically based on the relative magnitude of their Delta(f)G(0). In addition, the values of molar heat capacities at constant pressure (C-p,C-m) for PBDT congeners are calculated.