A theoretical study on the molecular and electronic structures and the enthalpies of formation of several polychlorinated dibenzo-p-dioxins, PCDDs, has been carried out using DFT at the B3LYP/6-31G(d) and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels of theory. The calculated molecular structures are planar in all of the studied PCDDs, in agreement with the available X-ray diffraction data. NBO charges, and HOMO and LUMO energies have also been obtained and discussed. The enthalpies of formation have been calculated through isodesmic reactions, and the values obtained have been compared with the few available experimental values and with values estimated by other authors using semiempirical methods or group additivity approaches.