The relative effects of seven additives (n2, he, co2, fly ash, khco3, nacl, and nh4h2po4) on extinction limits of coal dust flames have been studied in a 200-cm3 jet-stirred reactor. Conditions in the reactor are known to be comparable to those at the head of a coal dust explosion flame. Khco3, nacl, and nh4h2po4 are commercial fire retardants and are presumed to be chemically active, but test results support the presumption only for nacl and nh4h2po4. Curves of weight-of-additive to suppress the flame and temperature at extinction were obtained as a function of stoichiometry. For all but the nacl and nh4h2po4, 20% to 30% additive in the mixture of coal plus air plus additive is required for extinction of maximum combustibility. With nacl and nh4h2po4, the figures were 13% and 4%, respectively. The extinction mechanism is presumed to be chain termination of volatiles combustion. Extinction temperatures were found to be substantially independent of kind of additive, with essentially constant values in the fuel-rich region, and declining as fuel concentration decreased.