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A laboratory screening test of compounds for their fire inhibiting properties was set up by determining the limits of flammability of mixtures of fuel, (n-heptane) air, and the proposed extinguishing agent. The peak in the curve obtained by plotting data concerning the flammability of mixtures was considered as a measure of the flame extinction properties of the test compound. This hypothesis was later shown to be valid. Dibromodifluoromethane heads the list of the thirty-one compounds found to be more effective than methyl bromide on the basis of this test. When the comparisons were made on a weight basis instead of a volume basis only eight compounds were found to be more effective than methyl bromide. It was found that in a given homologous series of compounds, the effectiveness in fire extinction properties increases with an increase in molecular weight. No apparent relationship could be found between the fire extinction properties and molecular weight of compounds chosen at random. (Author)