The entrained fine fly ash particles carried into the post-combustion region and collected by air pollution control devices were capable of such surface-mediated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs) formation. The secondary formation of PCDD/Fs in the flue gas and ash from the municipal solid waste incinerator (MSWI) systems has attracted considerable public concern. The objective of this study was to examine reactions in fly ash from an municipal solid waste incinerator (MSWI) operated with metal chlorides and chlorinated aromatic precursors. When 10 wt% FeCl2, ZnCl2 and CuCl2 were added onto the fly ash medium, the PCDD/Fs concentration is 8.8, 25.5 and 369.5 ng/g, respectively. This indicates that the formation of PCDD/Fs was significantly increased by CuCl2. In the case of CaCl2 addition, the results showed that it acts in a minor role as a catalyst to promote the formation of PCDD/Fs. In addition, the results of adding HgCl2 has no transformation effect on the formation of PCDD/Fs in the residue and gas phases. Moreover, in an experiment using chlorinated aromatic precursors, the formation of PCDD/Fs was significantly increased by pentachlorophenol (PCP). This study suggests that the filter cake ash treatment should be studied extensively to control dioxin yields on the fly ash matrix.