This work investigates the pyrolysis of n-butylbenzene, which widely exists in transportation fuels and their surrogate mixtures. Both reactive and stable pyrolysis products were comprehensively detected with synchrotron vacuum ultraviolet photoionization mass spectrometry. Their mole fractions versus temperature were also evaluated at 30, 150, and 760 Torr. A kinetic model of n-butylbenzene pyrolysis was developed, and new data were used to validate the model. On the basis of the modeling analysis, the benzylic C-C bond dissociation that forms the benzyl radical and the propyl radical was found to be a key decomposition reaction of n-butylbenzene at all investigated pressures, whereas H abstraction provided increasing contributions with increasing pressure. Compared with small alkylbenzenes, such as toluene and ethylbenzene, n-butylbenzene demonstrates different pyrolysis characteristics and chemistry because of the existence of its long alkyl side chain. n-Butylbenzene has a higher pyrolysis reactivity and lower decomposition temperature regions, which inhibit the further decomposition of the benzyl radical and the formation of highly unsaturated C-2-C-4 products. As a result, conventional combination reactions between aromatic radicals and highly unsaturated C-2-C-4 species are only minor formation pathways for indene and naphthalene in nbutylbenzene pyrolysis, while fuel-specific pathways become crucial instead. Furthermore, combination reactions involving the benzyl radical and the phenyl radical are crucial for the formation of many PAHs, especially phenanthrene and fluorene. The results in this work reveal the strong influence of side-chain length on the pyrolysis chemistry of alkylbenzenes and indicate a further need for exploring the influences of other structural features.