We report a photoelectron spectroscopy and density-functional study on mono-rhenium oxide clusters: ReOn- and ReOn (n = 3, 4). Electron affinities of ReO3 and ReO4 are measured to be 3.53 +/- 0.05 and 5.58 +/- 0.03 eV. ReO3- is shown to possess a planar D-3h ((1)A(1)') ground state and ReO3 adopts a nonplanar C-3v ((2)A(1)) structure. ReO4- has T-d ((1)A(1)) structure, whereas ReO4 has C-s ((2)A') symmetry due to Jahn-Teller effects and contains a Re-O-center dot radical unit. Localized Re 5d electrons in ReO3- and ReO3 give rise to Re5+ and Re6+ centers, making them simple models for O-2 activation. (C) 2011 Elsevier B. V. All rights reserved.