The paper assesses extensive data of PCDD/F measurements on flue gas emissions from thermal processes, including, e.g. municipal solid waste incinerators (MSWIs), combustors of wood and industrial waste, coal fired powerplants and boilers, ferro and non-ferro processes. Numerous investigators have conducted laboratory experiments to assess the formation mechanisms of PCDD/F. The results, obtained from fixed-bed experiments, have been critically evaluated and indicate that de novo synthesis is the dominant mechanism in actual thermal processes where conditions that favour the precursor formation are not experienced. The analysis of PCDD/F profiles from the large scale thermal processes in general, and MSWIs in particular, supports the dominant role of the de novo synthesis, irrespective of the type of thermal process considered. The PCDF/PCDD ratio exceeds 1 and the degree of chlorination points towards the dominant presence of HpCDD and OCDD within the dioxin group, and of PeCDF, HxCDF and HpCDF within the furan group. Since real-time measurement of PCDD/F is impossible, the correlation of PCDD/F emissions with operating parameters and/or emission levels of other more easily measured pollutants could be a tool in predicting the PCDD/F formation levels. Data of Flemish MSWIs were used to statistically assess such correlations. From an evaluation of the data at a given operating temperature, misleading conclusions can be drawn. Only the effect of temperature is evident. After converting all data at a reference temperature of, e.g. 230 degrees C, PCDD/F concentrations achieve nearly constant values, irrespective of the values of other parameters, thus stressing that the major controlling parameter for the PCDD/F emission is the temperature of the ESP. The PCDD/F concentrations increase with temperature in the range up to 280 degrees C. The ESP temperature should be kept preferably between 180 degrees C and 200 degrees C, where de novo synthesis is reduced and where PCDD/Fs are increasingly adsorbed on the fly ash, in line with the standard temperature dependence of adsorption isotherms.