Laboratory investigations were carried out of the effects of several sulfur compounds of different types on the combustion of two pure hydrocarbons, viz., 2-methylpentane and isooctane, in the absence and presence of lead compounds. The sulfur compounds tend to promote combustion, in the order sulfur dioxide < thiophene < dipropyl sulfide < dipropyl disulfide although exceptions occur for unleaded isooctane. The inhibiting rather than promoting effect of some sulfur compounds on the combustion of isooctane is probably related to the small extent of preflame reactions of this hydrocarbon, as compared with 2-methylpentane, and to the use of relatively inefficient sulfur containing promoters. Organic sulfur compounds are more effective combustion promoters than sulfur dioxide. All sulfur compounds used in this work increase the consumption of the hydrocarbons, the percentage conversion depending on the temperature and on the type of combustion taking place. On the other hand, they decrease the proportion of 2-methylpentane converted to each of the readily identifiable major products. Studies, by EDAX, of the surface of the lead monoxide coating of the reaction vessels show that the metal oxide can accommodate more sulfur from sulfur dioxide than from organic sulfur compounds. Interpretations have thus been advanced of the effects of sulfur and lead compounds on the nature of the intermediate species involved in the underlying combustion reactions.