Abstract The effect of the reaction time and temperature on PCDD/F formation on a fly ash surface was studied. Carbon was used as a source for de novo synthesis. There is a great rise in PCDD/F formation between 2 and 4 hours annealing time. This is probably due to changes in the fly ash surface. The optimum temperature for formation was found to be around 350 °C, which is higher than the 300 °C found in other publications. Whereas the congener distribution changes as function of both reaction time and temperature, the isomer distribution shows little reaction time or temperature dependence.