About 100 tests of flammability were carried out with h2, ch4, and CO in mixtures with air using the federal Bureau of Mines 12-ft- diameter spherical pressure vessel as an explosion test chamber. The lechatelier approximation gave slightly conservative estimates of the limits of downward flame propagation (onset of significant pressure rise) using the following measured values for the individual gases, in percent: h2, 8.5; Ch4, 5.25; Co, 13.0. No justification could be offered for using the lechatelier law for upward flame propagation limits and indeed reported data suggest that calculated values would not be conservative. The observed upward limit for methane-air mixtures (4.85 pct) was less than the widely accepted lean limit for this fuel; those for lean hydrogen- air mixtures and carbon monoxide-air mixtures did not differ from the lowest accepted values. Diffusional effects on the propagation of single, binary, and ternary fuel mixtures are discussed.