Niobium oxide cluster ions are produced by a laser-induced plasma source. The cluster distribution, collision-induced dissociation (CID), and cluster reactivities are studied using a triple-quadrupole mass spectrometer. CID experiments on the cluster ions Nb3O7-9+, Nb4O9-11+, and Nb5O12+ reveal that their building blocks are Nb2O5, NbO2+, NbO3, Nb3O7+, O, and O-2, whereby the cluster stoichiometry is assigned to have the general form (NbO3)(m)(NbO2)(n)(O)(0-4)(+). The trends in the ionization potentials of these species are estimated in terms of the CID fragments produced, Nb3O8-9+ and Nb4O11+ cluster ions evidently form via the adsorption of one oxygen atom or molecule onto the cluster surface. Nb3O7+, Nb4O9+, and Nb5O12+ have strong reactivities to abstract an oxygen atom from oxygen-containing molecules and adsorb small hydrocarbons at near thermal energies. In particular, the reactivity of the oxygen atom or molecule in the oxide clusters Nb3O8-9+ and Nb4O11+ is consistent with our suggestions that it has a radical oxygen character.