A kinetic model for the oxidation of mixtures of n-heptane/iso-octane, the primary reference fuels for octane rating in spark-ignited internal combustion engines, has been written by using the system of automatic generation of mechanisms developed in Nancy. The relevant mechanism involves 647 species and includes 2386 reactions. The concentration profiles predicted by this model have been compared with experimental results obtained in a perfectly jet-stirred reactor in a temperature range from 580 to 1150 K, which includes the negative temperature coefficient region. A good agreement has been obtained for both the conversion of reactants and the distribution of the products formed. Reaction rates analyses have been performed for both compounds at 650 and 1100 K. A sensitivity analysis and a study of the influence of the structure of each molecule on the preferential reaction channels have been carried out and have enabled us to explain the changes in reactivity with the composition of the mixtures studied.