Structures of cobalt oxide cluster cations have been investigated by ion mobility mass spectrometry. When the ions were injected into a drift cell with 250 eV kinetic energy, (CoO)(+)(n) and ConOn-1+ were observed. Collision cross sections were experimentally determined for (CoO)(+)(2-7) and ConOn-1+ (n = 5-7). Orientation-averaged collision cross sections were calculated for optimized structures of these ions obtained in quantum chemical calculations. By comparison between experimental and theoretical cross sections, (CoO)(+)(3-5) ions are found to have monocyclic-ring structures. By contrast, (CoO)(+)(6;7) have compact tower structures. Therefore, structural transition from the ring to compact structures occurs at (CoO)(+)(6). (C) 2013 Elsevier B. V. All rights reserved.