Electron impact ionization of n-octane over an energy range of 10-70 eV and the subsequent ion-molecule reactions with the parent molecule have been studied using Fourier-transform mass spectrometry. Molecular ion and fragment ions C-1(+)-C-6(+) are produced from the electron impact with a total ionization cross section of 1.4 +/-0.2x10(-15) cm(2) between 60 and 70 eV. C3H7+ is the most abundant ion at most of the ionizing energies with the exception for E less than or equal to 16 eV where C6H13+ and C6H12+ are the most abundant. Among the fragment ions only C4H7+ and smaller ions react readily with the parent molecule, primarily producing C5H11+ and C4H9+, with rate coefficients of 0.32-2.4x10(-9) cm(3) s(-1). Essentially all of the ions, including the molecular ion and the large fragment ions, undergo decomposition upon collision with neutral molecules after they are kinetically excited to an energy range of 1-5 eV, forming a variety of small hydrocarbon ions. Many of the decomposition product ions in turn are capable of further reacting with n-octane. Isotope reagents have been utilized in experiments to probe the type of the ion-molecule reactions studied. (C) 2001 American Institute of Physics.