Structures of the most relevant 'cluster ions' (C4H5N2+, C4H7N2+, C5H7N2+, C6H10N3+) produced in the high and medium pressure chemical ionization of acetonitrile are here proposed. Through a joint approach, whereby experimental data have been compared with ab initio calculations, we have elucidated their structures and evaluated their thermochemical stabilities. Up to now, only elemental formulas have been proposed for such ionic species and often they have been tacitly assumed as electrostatically bound adducts. We report here that they arise from covalent bond rearrangements leading to intriguing oligomeric covalent structures. (c) 2005 Elsevier B.V. All rights reserved.