Physical and chemical characteristics of fly ash samples from thirteen U.S. MSW incinerators were tested for correlations with synthesized PCDD concentrations. The fly ash was previously extracted to remove all PCDD. Pentachlorophenol was then passed over the samples in flowing nitrogen at 300-degrees-C. Strong correlations may indicate catalytic activity for PCDD synthesis. Copper, potassium, sodium, sulfur, and zinc were positively correlated and aluminum was negatively correlated. These elements closely match those identified in our earlier studies using PCDD concentrations of the raw fly ash. No correlations were noted for carbon, chlorine, or physical parameters such as surface area or particle size distribution.