Sludge decomposition gases (SDG) have been identified as potential PCDD/F-inhibitors, since they contain high concentrations of NH3 and SO2 as well as various unidentified N- and S-compounds. The effects of sludge dosage, its treatment temperature and oxygen content on PCDD/Fs suppression were observed during test runs using a model fly ash (MFA) system, generating high chlorinated PCDD/Fs. The experiments were carried out in two parts: a first set with SDG of undefined composition, and a second with injection of NH3 and SO2 combined in three different concentration ratios. It was observed that the SDG evolving from dried sludge (DS) could suppress the formation of 2,3,7,8-substituted PCDD/Fs even with very high efficiency (> 99%), both in weight units and in I-TEQ units. Such suppression was somewhat stronger for PCDD than PCDF. It was also found that 300°C might be a suitable sludge decomposition temperature when using SDG as suppressant in PCDD/Fs inhibition. Moreover, the oxygen content only slightly influenced upon the suppression capabilities. The results revealed that SO2 was more effective than NH3 in suppressing PCDD/Fs formation, with suppression efficiencies of 61.9% and 38.6%, respectively. Remarkably, the formation of OCDD/Fs was inhibited least and even further formation could occur when both NH3 and SO2 were injected together as simulated flue gas. The results exposed that NH3 and SO2 might be the primary PCDD/Fs suppressants resulting from mild thermal decomposition of sludge. However, further parameters should be evaluated and adjusted before SDG could be used in large-scale applications.