Thermal dissociation of gas phase iron oxide cluster ions, FenOm' (n = 2-6), was observed by mass spectrometry. The dissociation processes were investigated by temperature-programmed desorption (TPD) measurements for different sized clusters. Oxygen molecules were found to be released from the cluster ions. The threshold energy required for dissociation, determined by analyzing TPD, was compared with the energies obtained by experiments of collision-induced dissociation and by calculations of density functional theory. The agreement of the energies indicates that the oxygen atoms bonded to the terminal site of clusters are more readily released into the gas phase than those in the bridge site. (C) 2015 Elsevier B.V. All rights reserved.