The classical theory of atom ionization by electron impact, developed by Bethe [H. Bethe, Ann, Phys. 4 (1930) 325], has been extended to calculate the ionization cross sections of vibrationally excited molecular oxygen in the ground state and in the low lying singlet states. Results show the influence of vibrationally and electronically excited states in reducing the threshold energy of the cross section as compared with the ground state as well as in enhancing the maximum of cross section. (c) 2006 Elsevier B.V. All rights reserved.