The pyrolysis experiments of isooctane were conducted in a flow tube at the temperature from 848 to 1123 K at 30 Torr and from 723 to 998 K at 760 Torr. The pyrolysis intermediates and products were photoionized by synchrotron vacuum ultraviolet and detected by a reflectron time-of-flight mass spectrometer. A pyrolysis model was developed to compare the results with the experimental data to improve the combustion mechanism and to clarify the important pyrolysis pathways of isooctane. The model includes 321 species and 1569 reactions. It can be revealed from the rate of production and sensitivity analyses that in the initial decomposition of isooctane, unimolecular dissociation reactions are more important than H-abstraction reactions. The reaction pathways and branching ratios are affected by the change of pressures. The reaction networks of C-5-C-2 unsaturated hydrocarbons are dominated by the production of isobutene and propene via various reaction channels. Small amounts of benzene and fulvene are formed from the reactions of smaller radicals and molecules.