Toxic chlorinated dibenzo-p-dioxins are known to be formed in incinerators that burn municipal refuse. These compounds were synthesized by surface-catalyzed reactions on fly ash particulates taken from incinerators. Dioxins were produced catalytically from chlorinated phenol precursors, from non-chlorinated compounds that were chemically dissimilar to dioxins, and from reaction of phenol with inorganic chlorides. The relative amounts of dioxins formed from [13C6]pentachlorophenol with different fly ashes that had been cleaned of all organic compounds corresponded well with those amounts originally found on the samples as received from the incinerators. The optimum temperature range for the formation of dioxins from pentachlorophenol was 250 degrees to 350 degrees C.