Thermal dissociation of cationic niobium oxide clusters (NbnOm+) was hivestigated by gas phase thermal desorption spectrometry. The dominant species formed at 300 K were NbnO(5/2)n+p+ (n = 2, 4, 6,...; p = 0, 1, 2,...) and NbnO((5/2)n-1/2)+q+ (n = 3, 5, q = 0, 1, 2,...). At higher temperatures, the more oxygen-rich clusters were observed to release O-2. However, the desorption of O-2 from NbnOm+ was found to be insignificant in comparison with VnOm+ because Nb tends to have a +5 oxidation state exclusively, whereas V can have both +4 and +5 oxidation states. The propensity for the release of O atoms was manifested in the formation of NbnO(5/2)n-1/2 from NbnO((5/2)n-1/2)+1+ for odd values of n, whereas Vb(n)O(((5/2)n-1/2)+1)(+) released O-2 molecules instead. The energetics of the O and O-2 release from the Nb and V oxide clusters, respectively, was consistent with the results of DFT calculations.