Polybrominated dibenzo-p-dioxins and furans (PBDDs/PBDFs) and the mixed brominated/chlorinated congeners (PXDDs/PXDFs) are much less understood than their chlorinated analogues (PCDDs/PCDFs). In particular, there is very little experimental data on their physicochemical properties and formation behavior. The practical purpose of the study was to obtain a consistent set of thermodynamic values for PBDDs/PBDFs and PXDDs/PXDFs, and, using these values, to consider their formation behaviors in the flue gas from flame retardant combustion. Thus, for all known PBDDs/PBDFs and 339 PXDDs, we calculated various thermodynamic properties (heat capacity, entropy, enthalpy and Gibbs free energy of formation) in the gaseous state using density functional theory (DFT). Using their calculated free energies of formation, we then conducted a thermodynamic simulation of their formation in combustion flue gas.