Polychlorinated diphenyl ethers (PCDEs) are a focus of current environmental concern. The formation of PCDEs was investigated using laboratory-scale flow reactors under air and under nitrogen at 350 degrees C. The results show the amount of PCDEs formed from the condensation of pentachlorophenol (PCP) and 1,2,4,5-tetrachlorobenzene (1,2,4,5-Cl(4)Bz) was lower with oxygen than without oxygen, and the PCDE homologue distribution was different under both atmospheres. The presence of oxygen influenced the process of PCDE formation from precursors on the fly ash surface. Furthermore, 2,2'.3,4,4'.5,5'.6-octachlorodiphenyl ether (OCDE) is the dominant PCDE homologue product formed on the surface of SiO2, whereas decachlorodiphenyl ether (DecaCDE) was the predominant product formed on the SiO2/FeCl3 and SiO2/CuCl2 surfaces. The PCDE homologue distribution shifted toward the more chlorinated species in the presence of FeCl3 and CuCl2. The simultaneous formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) was also observed during the formation of PCDEs from heterogeneous reactions of precursors. These findings suggest that the emission of PCDEs from municipal waste incineration plants should be considered, and careful and sophisticated control is required to prevent the formation of pollutants. (C) 2010 Elsevier B.V. All rights reserved.