We examined the influence of the properties of macromolecular carbon on thermochemical de novo synthesis of toxic chlorinated aromatic compounds (aromatic-Cls), such as polychlorinated dibenzo-p-dioxins (PCDDs), furans (PCDFs), biphenyls (PCBs), and chlorobenzenes (CBzs). Six types of macromolecular carbons were prepared. Some activated carbon samples were modified by chemical solvents, such as nitric acid, hydrogen peroxide, sulfuric acid, and urea solutions, to change the properties of the activated carbon. We characterized six macromolecular carbons by surface area (8.8–1540 m2/g), free radicals (Ns = not analyzed to 67.0 × 1019 g–1, g = 2.0011–2.0098), and functional groups (O–H, C–H, C=O, C=C, C–O, and C–OH). Concentrations of aromatic-Cls at 300–400°C, such as PCDDs, PCDFs, PCBs, and CBzs, were clearly influenced by the type of macromolecular carbon. Their distribution between the ash/gas phases implied that increasing the surface area mainly enhanced the adsorption capacity of macromolecular carbon, while increasing the number of free radicals mainly enhance the reaction activity of macromolecular carbon. The PCDD/PCDF ratio suggested that various modifications of macromolecular carbon contributed to the generation of PCDDs in addition to the catalytic behavior of copper. Under most conditions, the surface area of macromolecular carbon did not have a strong correlation with the generation of PCDDs, PCBs, and CBzs, but it did show have a correlation with the generation of PCDFs. One of the destructive effects of aromatic-Cls resulted from the free radicals in macromolecular carbon. The C=O bond (ca. 1720 cm–1) functional group in macromolecular carbon had no strong correlation with the generation of aromatic-Cls, because free radicals had a destructive effect. The functionalities of ether (C–O) or phenolic OH (C–OH) in macromolecular carbon were causative factors in the generation of oxygen-containing aromatic-Cls, such as PCDDs and PCDFs.