Formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) from carbon on incinerator fly ash was investigated using HCl and Cl-2 as a chlorinating agent. The formation proceeds roughly at an equal rate with both gases. These results do not support the theory that chlorination reactions on the fly ash surface take place through free Cl-2 generated by the Deacon reaction. Once formed, PCDD/F remain adsorbed on the fly ash surface until ca. 350 degrees C; at ca. 400 degrees C, the major part of these compounds has evaporated from the fly ash to the gas phase.