Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) were determined in samples of ash residue collected from various parts of the incinerators at two large UK municipal solid waste incinerator (MSWI) plants. The results confirmed that conditions in the heat recovery system favoured the formation of PCDD/Fs and that these were removed from the flue gas by activated carbon and fly ash in the fabric filters downstream of the incinerator. Small concentrations were observed in the bottom ash due to the exposure of the ash to temperatures exceeding 600 degreesC, followed by the rapid quenching of the hot ash. The patterns of PCDD/F congeners were also determined and recorded. As these patterns vary according to local plant conditions and combustion sources, the data can be used for the tentative assessment of the accountability of current MSWIs for the presence of PCDD/Fs in the environment. A mathematical model was developed for predicting concentrations of PCDD/F based on the fundamental governing equations for formation of PCDD/Fs by the De Novo synthesis mechanism in the heat recovery unit and their removal by the fabric filter. The four important factors influencing formation of PCDD/Fs by De Novo synthesis are; temperature (250 to 400 degreesC, with a peak formation rate at about 300 degreesC), concentration of oxygen in the gas stream, and concentrations of carbon and copper in the fly ash. A key feature of the mathematical model is, therefore, its ability to predict PCDD/F concentrations based on integrating the effect of these parameters. Application of the model to data from MSWI plants gave predicted PCDD/F concentrations in good agreement with the experimentally measured results.