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Since the water is the most famous, non-ozone depleting and effective agent in fire fighting, a water mist is noticed as favorable one of halon alternatives. There are many studies on a fire extinguishing system of the water mist. However, further studies are required on a fire extinguishing effect of the mist for its better application. Except hydrodynamic properties of the mist in extinguishing fire, it is important to know fire extinguishing efficiency of water vapor that has been assumed to have only thermal effect as a fire suppressant. There is not enough data about such fire extinguishing efficiency of the vapor. In the study, reducing effect of the vapor for flammable regions of methane and propane was measured at 80Â°C. The authors used a tubular burner system to measure the limits under well-defined conditions of flame stretch and temperature. The effect was compared with a reducing effect of carbon dioxide at 80Â°C and 25Â°C. Experimental results showed that fire extinguishing effect of the vapor is the same as carbon dioxide for both methane and propane. Since calculated flame temperatures of the mixtures with the vapor are apparently higher than the temperatures of mixtures with carbon dioxide at extinction conditions, the chemical suppression efficiency of the vapor is superior to that of carbon dioxide. This result is attributed to the larger third body effect of water vapor than carbon dioxide in the reaction H + O2 + M = HO2 + M. From the result and thermal properties of water, we can estimate required mass per unit volume of water vapor to extinguish n-heptanes flame. The required mass per unit volume is 162 g/m3, and it is about 0.6 times of required mass per unit volume of halon 1301. The facts show that water mist is a high-performance agent, since the water mist has large latent heat.