The inhibitory mechanisms of alkaline compounds on the formation of chlorinated aromatic (aromatic-Cl) compounds in postcombustion fly ash from thermal processes such as municipal solid waste (MSW) incineration are not fully understood. Here, we report quantitative and X-ray spectroscopic evidence that deactivation of metal chloride promoter activity by alkaline compounds inhibits the formation of aromatic-Cl compounds. The formation of aromatic-Cl compounds such as chlorobenzenes and polychlorinated biphenyls in real MSW fly ash was inhibited by the addition of NaOH, Ca(OH)(2), or NaHCO(3), either dry or in solution, with the fly ash. With optimal conditions, the formation of aromatic-Cl compounds was inhibited by more than 95% in comparison with formation in reheated raw MSW fly ash. We prepared simplified model fly ash samples to estimate the influence of alkaline compounds on trace Cu, Fe, Pb, and Zn chlorides, which strongly promote aromatic-Cl compound formation. More than 99% inhibition was observed in some model samples. Cl K-edge X-ray absorption and X-ray diffraction provided clear evidence of promoter deactivation, as NaOH or NaHCO(3) changed to NaCl, and Ca(OH)(2) changed to CaCl(2) or CaClOH by reaction with the metal chlorides. NaOH was the most reactive and useful of the three alkaline compounds tested. We recommend deactivation of metal chlorides as an environmentally friendly method of inhibiting the formation of aromatic-Cl compounds, with the added benefit of changing the alkaline compounds and metal chlorides into harmless chemicals such as NaCl and metal oxides.