The conditions for accurate determination of the limit mixtures for propagation of flame having been established, the effect of the addition of 0$\cdot $5% of such promoters and inhibitors as ethyl nitrate, diethyl peroxide, nitrogen peroxide, ozone, ether, acetaldehyde, ethyl alcohol and methyl iodide on the upper and lower limits of hydrogen-air, methane-air, n-butane-air (and propagas-air) mixtures was investigated. It is shown that the effects obtained are accounted for if the heat which the additive can contribute to the total heat available in the burning of the mixture is included. The only distinct promoting effect was that of ethyl nitrate on the upper limits of n-butane and propagas. Methyl iodide had a definite inhibiting effect except with hydrogen mixtures at the lower limit. The appearance of the flames has been carefully recorded as it gives some indication of the nature of the combustion process. The general inference is that pre-flame reactions have little influence on the limit mixtures, and that those mixtures are mainly determined by the heat available and consequently the temperature of the flame; inflammation being mainly caused by the radicals provided by the flame. The limit mixtures of ethyl nitrate and nitrite were measured. The effect of replacement of oxygen by nitrous oxide on the hydrogen limits, and the effects of small additions of lead tetraethyl and of 0$\cdot $5% ethyl borate on n-butane limits are also noted.