We studied an effective repulsive state of O-2(-) through collision induced dissociation (CID) with Ar at an impact energy of 15 keV. The measured lab frame kinetic energy distribution for O- fragment is used to deduce an effective repulsive potential energy state of the molecular anion using the reflection method. The experimentally derived potential energy state is used to evaluate the rigorous and difficult exercise of computing the electronic states of the negative molecular ion. The deduced repulsive state appears close to the computed (2)Pi(u) excited state of O-2(-) and is attributed to be a better representation of this curve. (c) 2006 Elsevier B.V. All rights reserved.