The effect of sulfur dioxide (SO2) on formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) has been studied in an entrained-flow reactor (EFR) under simulated waste combustion conditions. A chlorination model based on conditional probability was employed to evaluate the homologue patts of PCDDs and PCDFs. Results revealed that the presence of SO2 did not change the formation pathway although SO2 suppressed PCDD/F formation. The model prediction of PCDF showed good agreement with the experimental data (R=0.95) while the prediction for PCDDs was not as good. This may be explained because potential chlorination pathways play a significant role in PCDF formation, while PCDDs are mainly formed through condensation reactions. Furthermore, the result indicated that the steric hindrance during formation has more effects on PCDD than on PCDF due to the symmetric molecule structures of PCDDs.