ConO- (n = 1-3) clusters were investigated with photoelectron spectroscopy and density functional calculations. The vertical detachment energies (VDEs) of ConO- (n = 1-3) were measured to be 1.54 +/- 0.04, 1.43 +/- 0.08, and 1.42 +/- 0.08 eV respectively from their photoelectron spectra. The electron affinity and term energy of CoO were determined to be 1.54 +/- 0.04 eV and 0.31 +/- 0.04 eV respectively based on the vibrationally resolved photoelectron spectrum of CoO- and theoretical calculations. The structures of ConO- (n = 1-3) were determined by comparison of photoelectron experiments and calculations. The analysis of molecular orbitals shows that the HOMOs of ConO- (n = 1-3) cluster anions are mainly localized on the Co atoms. (C) 2013 Elsevier B. V. All rights reserved.