Heat capacities and entropies for 76 polybrominated dibenzo-p-dioxins (PBDDs) in the gas state at 298.15 K and 101.325 kPa have been computed using the density functional theory (B3LYP/6-31G(d)) with Gaussian 98. Based on the output data of Gaussian, three methods were employed to calculate enthalpies and Gibbs energies of formation of the 76 PBDDs in the gaseous state at 298.15 K and 101.325 kPa. To assess the three methods, thermodynamic properties of 16 compounds were first calculated by B3LYP/6-31G(d) and compared with reference values. For predicting the enthalpies of formation of the reference compounds, method 2 has the smallest average absolute deviation from the experimental data. All values for the heat capacity, entropy, enthalpy, and energy of formation of the 76 PBDDs increase as the number of substituted bromines increases. For isomers of tetrabromodibenzo-p-dioxins, 1,3,6,8-TeBDD, 1,3,7,8-TeBDD, 1,3,7,9-TeBDD, and the most toxic compound 2,3,7,8-TeBDD are more stable than the others and easier to form during the formation process.