The effect of a large variety of organic substances on the upper inflammability limit of hydrogen-air mixtures has been studied. The effects of the different substances have been compared, and it has been shown that the oxygen-using capacity of the added substance is most important in governing the effect on the limit. However, certain special atoms and groups (e.g. iodide) appear to have appreciable specific, though secondary, effects. The effect of methyl alcohol on the products of combustion and on the limit flame temperatures has also been studied. It appears that the organic substances exert a dual effect on hydrogen limit flames and that, when only small amounts are present, they interfere with the hydrogen-oxygen reaction by removing hydrogen and oxygen atoms and hydroxyl radicals. The upper limits of hydrogen-carbon monoxide-air mixtures have been studied and the effect of varying amounts of n-propyl alcohol, toluene and iso-propyl iodide on them. The effect of water on the carbon monoxide-air upper limit has also been examined. All these results have been considered in an attempt to obtain more information regarding the mechanism of flame propagation, the factors governing the composition at the limits and the limit flame temperatures which have been shown by Egerton & Powling to be very important. The results suggest that, in the systems studied, the reactions that are most important in governing the limits are the branching chain reactions. The part played by radical diffusion has also been discussed.