Thermal dissociation of cationic niobium oxide clusters (NbnOm(+)) was investigated 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 O2. However, the desorption of O2 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 VnO((5/2)n-1/2)+1(+) released O2 molecules instead. The energetics of the O and O2 release from the Nb and V oxide clusters, respectively, was consistent with the results of DFT calculations.