Abstract A new approach to the study of combustion phenomena in a flow-type apparatus consists of measuring exothermicities of the complex reactions taking place when a mixture of a hydrocarbon, oxygen and nitrogen is heated to various temperatures up to and including the auto-ignition temperature. The resultant plot of such data, i.e. exothermicity versus temperature, has been designated as a ‘thermal profile’. A thermal profile can delineate all the typical combustion regions, i.e. precool-flame, cool-flame, negative temperature coefficient and auto-ignition. These thermal profile studies include variations of hydrocarbon concentration, oxygen concentration, residence time and hydrocarbon structure. The dramatic influence of reactant ratio on combustion phenomena becomes apparent from these data. A correspondence was found between the thermal profiles and the octane numbers for the respective hydrocarbons. Some interesting applications of this new approach include studies of hydrocarbon blends, and the effects of certain antiknocks on n-hexane-oxygen reactions.