NaCl was added to a solid waste incinerator fly ash to study its role in the formation of polychlorinated dibenzop-dioxins (PCDD) and dibenzofurans (PCDF). Parameters that varied were as follows: chloride concentration, oxygen concentration, temperature, and time. None of these reaction conditions resulted in an increase in PCDD/F formation, compared to undoped ash. An essentially zeroorder behavior was observed for NaCl. When added to aqueous extracted ash, from which the soluble inorganic chloride had been removed before the addition, formation rates increased, with an optimum for [Cl -] ) 0.4 mmol/g (1.5 wt %). These results suggested that NaCl acted only as a chlorinating reagent on fly ash when no other chlorine sources were available. Surprisingly, only PCDF was formed on the aqueous extracted ash after addition of NaCl. Apparently, the PCDD formation catalyst had been removed by the water.