Recent efforts have been made to establish readily measurable surrogate compounds, such as chlorophenols, for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), that would enable plant operations to limit formation. Despite the extensive studies conducted on PCDDs/Fs formation from chlorophenols, very few studies have been carried out in real combustion conditions with a realistic concentration of precursors and the presence of water. In the present study, low (10(-9) M), stable concentrations of chlorinated phenols that are representative of concentrations of such compounds in municipal waste incinerator (MWI) raw flue gas were used in experiments investigating the formation of PCDDs/Fs over fly ash. Different mixtures of the chlorophenols (CPs) studied (2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 2,3,4,6-tetrachlorophenol) were passed through a bed of oxidized fly ash (carbon-free) and glass beads with and without the presence of water. The chlorophenol reactants used in this study were found to favor PCDD over PCDF formation. The presence of water was observed to considerably reduce the yields of all PCDD/F formed (< 0.3% phenol conversion). The PCDD homologue and isomer distributions were not affected by the presence of water, unlike the PCDF compounds. The major PCDD homologue groups formed were tetra- and penta-, both with or without water in the gas stream. The major PCDF homologue groups were mostly the lower chlorinated ones in the experiments performed in the presence or absence of water. These results contribute to the understanding of PCDD/Fs formation in realistic combustion conditions, including very low concentrations of precursors and the presence of water in the flue gas.