The thermodynamic properties of 39 polybrominated diphenyl ethers (PBDEs) in the ideal gas phase have been calculated using Gaussian 03 on the B3LYP/6-31G(d)//B3LYP/6-31G(d) level. Their thermodynamic and other physicochemical properties show a strong dependence on the bromine substitution pattern. The PBDE congeners' enthalpies of formation increase with an increasing number of bromines. The thermodynamic properties of congeners with the same number of bromines also show dependence on the bromine substitution pattern, especially for ortho-substituted congeners. PBDE congeners with one phenyl ring fully brominated, such as 2,3,4,5,6-PeBDE, 2,3,4,4',5,6-HxBDE, 2,2',3,4,4',5,6-HpBDE, and 2,3,3',4,4',5,6-HpBDE, were found to be the least stable among the analogues. The effects of bromine substitution pattern have been quantitatively studied by group additivity method (GAM) based on the output of the theoretical calculations. The results of the GAM were consistent with theoretical calculations, proving that theoretical calculations are reliable. Furthermore, the GAM model can be used to predict the thermodynamic properties for all of the 209 PBDE congeners.