A detailed experimental study of the nine isomers of heptane has been performed in a rapid compression machine. Our interest lies in determining the role of molecular structure of the C7H16 hydrocarbons on the rate of combustion of the various isomers. Ignition delay times were measured, and their dependence on the reaction conditions of temperature and pressure was studied, and in this way comparative reactivity profiles of the different isomers were obtained. Stoichiometric fuel and ‘air’ mixtures were studied in each case, at compressed gas pressure of 10, 15, and 20 atm for the n-heptane study, and at 15 atm for all other isomers, in the compressed gas temperature range of 640–960 K. Characteristic negative temperature coefficient behaviour was observed for each of the isomers. The more branched isomeric forms of heptane exhibited reduced reactivity, which correlated with the research octane number. In addition, the influence of fuel structure on burn rate was also studied. It was found that, similar to overall reactivity, the burn rate decreased with increasing octane number.