Temperature-programmed desorption (TPD) experiments were performed on gas-phase manganese oxide cluster ions, namely, MnnOm+ (n = 3-20) and MnnOm- (n = 3-18). These cluster ions were prepared by laser ablation of a manganese rod in the presence of oxygen gas, and their composition was investigated using mass spectrometry. The composition of MnnOm +/- distribution lies above the m = (4/3)(n) line. When the cluster ions were heated to 1000 K, Mn(n)Om(+) (m = (4/3)n + delta, with d = -1, 0) and Mn(n)Om (m = (4/3)n + delta, with d = 0, 1) was found to be the predominant species, formed by thermal dissociation. These experimental findings indicate that the nascent manganese oxide clusters comprise robust MnnOm +/- (m/n approximate to 4/3) and weakly bound excess oxygen atoms. On the basis of the TPD experiments, the oxygen-molecule release was identified as the main dissociation channel. The temperature dependence of O-2 desorption was found to be similar among the clusters with the same oxygen excess or deficiency regardless of the number of Mn atoms. The threshold energy of O-2 desorption was estimated for Mn4Om+ (m = 6-11) and compared with bond dissociation energies calculated by density functional theory.