Electron impact ionisation of iso-octane is studied using mass spectrometry. Cross- sections for the formation of fragment ions are measured between 17 and 79 eV. C4H9+ is the most abundant ion, C8H18+ is completely absent. A total ionisation cross-section of 2 x 10(-15) cm(2) is found at 79 eV. Ab initio calculations show that the principal ionic species observed, C4H9+, C4H8+, C7H15+, C3H6+ and C3H11+, may be issued directly from iso-octane by simple C-C bond split 9 8 15 h processes or with H atom rearrangements. Ionisation of the two major neutral species issued from bond split, C4H9 and C4H10, can lead to the principal ions observed. (C) 2003 Elsevier Science B.V. All rights reserved.