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To explore the potential risk of increased CO with halon replacements in fires, methane-air diffusion flames doped with suppressants were studied numerically. The suppressants investigated are nitrogen, H FC-23 and HFC-227ea as w ell as Halon 1301. When Halon 1301 is doped in the air side of the counterflow diffusion flame at constant oxidizer and fuel flow velocities, little change is observed in the production rate of CO. An addition of nitrogen causes reduced CO production because of decreased fuel consumption, while an addition of HFC-221ea causes a significant increase in both the maximum CO mole fraction and its production rate. CO in the HFC-227ea-doped diffusion flames is formed in two different regions, the methane oxidation region and an additional region on the oxidizer side of the flame, in which CO is produced via the oxidation of the suppressant. When normalized by the total amount of carbon released in the flame, the CO production rate is almost constant with variations of suppressant and its concentration. The increased CO production with HFC-227ea is attributed to the excess supply of carbon into the flame.