Heat capacities and entropies for 76 polybrominated dibenzo-p-dioxins (PBDDs) and 136 polybrominated dibenzofurans (PBDFs) in the gas state have been computed using the density functional theory. Based on the output data of Gaussian, three methods were employed to calculate enthalpies and Gibbs energies of formation of PBDDs and PBDFs in the gaseous state at 298.15 K and 101.325 kPa. To assess the three methods, thermodynamic properties of 16 brominated arenes compounds were first calculated and compared with experimental values. Among the three methods used, 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 formation process. Comparing with PBDDs, the formation enthalpies and Gibbs energies of PBDF isomers are more variable. The formation enthalpies and Gibbs energies of isomers which have bromine substitutions in 1 and 9 positions are much higher than those of the others.