In several accidents (fires, explosions) involving electrical systems containing polychlorinated biphenyls (PCBs) or benzenes (PCBZs), the formation of polychlorinated tricyclic aromatic compounds such as polychlorinated dibenzofurans (PCDFs), dibenzo-p-dioxins (PCDDs) and biphenylenes (PCBPs) was observed. These findings were not surprising in light of our previous studies on the formation of PCDFs and PCDDs from the thermolysis (500-750 degrees C in presence of air) of PCBs or PCBZs. In these experiments we identified three main and a fourth minor reaction pathway leading to PCDFs from PCBs. The three main reactions involved the loss of ortho-Cl2, and the losses of ortho-HCl with and without a 2,3-chlorine shift. The fourth, minor reaction route found with some PCBs, involved the loss of ortho-H2. These reaction pathways were intramolecular cyclizations via oxidation to phenolic compounds. Thermolysis of commercial PCBs (Aroclor 1254 and 1260) yielded the same PCDF isomers as expected from the main PCB components via these four reaction pathways; the isomers formed included the toxic 2,3,7,8-substituted tetra-, penta- and hexa-CDFs. Thermolysis of PCBZs involved complex condensation reactions with multiple rearrangements leading to PCDFs as well as PCDDs. Again oxidation to phenolic products was involved but the reaction routes are not yet fully elucidated. The results of samples from transformer and capacitor accident sites showed very complex PCDF mixtures with 2,3,7,8-substituted isomers as main components and originating from the PCBs in the transformer liquid. The unusual presence of PCDDs in samples from the Binghamton accident originates from PCBZs present in transformer fluid of that specific installation, thus confirming our previous thermolysis findings.(ABSTRACT TRUNCATED AT 250 WORDS)