The molecular structures of hydrocarbon fuels are known to have a substantial impact on their combustion properties. However, the relationship between the fuel structure and thermal decomposition intermediate products, which determine the global combustion behaviors, is not as well known. In this study, four octane isomers, n-octane, 2,5-dimethylhexane, 2,2,4-trimethylpentane (iso-octane), and 2,2,3,3-tetramethylbutane, are selected as the model compounds to illustrate the distinct product generation in the oxidative pyrolysis of large hydrocarbons with substantially different molecular structures. Both experimental and kinetic analysis show that all the octane isomers lead to the formation of a similar group of stable products, with the major ones being ethene, methane, propene, and isobutene. The distributions of these products vary from fuel to fuel; n-octane produces primarily ethene, while isobutene formation increases with increasing branching in the fuel molecular structure. The most branched isomer, 2,2,3,3-tetramethylbutane, produces predominately isobutene. Lumped, two-step reaction schemes are proposed for each octane isomer. Together with a detailed foundational fuel chemistry model, it is shown that the reaction models accurately predict the formation and subsequent consumption of the intermediate products for all octane isomers studied.