The　electronic　and　structural　properties　of　a　series　of　triniobium　oxide　clusters,　Nb3On-　and　Nb3On　(n　=　3-8),　are　investigated　using　photoelectron　spectroscopy　(PES)　and　density　functional　theory　(DFT)　calculations.　PES　spectra　are　obtained　for　Nb3On-　(n　=　3-8)　at　various　photon　energies　and　are　used　to　compare　with　the　DFT　calculations.　A　trend　of　sequential　oxidation　is　observed　as　a　function　of　O　content　until　Nb3O8-,　reaching　the　highest　oxidation　state　of　Nb.　DFT　calculations　are　performed　to　search　for　the　lowest　energy　structures　for　both　the　anionic　and　neutral　clusters.　For　Nb3O3-,　the　three　O　atoms　are　shown　to　prefer　the　bridging　sites　of　a　triangular　Nb-3,　leading　to　two　nearly　degenerate　cyclic　structures　of　C-s　symmetry.　The　next　three　O　atoms　from　n　=　4-6　each　occupy　a　terminal　site　directly　bonded　to　Nb,　resulting　in　a　symmetric　Nb3O6-　with　C-3v,　symmetry　and　a　low-lying　isomer　of　C-s　symmetry.　The　seventh　O　atom　is　bonded　to　two　Nb　atoms　forming　a　double　bridge,　whereas　the　eighth　O　atom　is　bonded　to　a　terminal　site　so　that　in　Nb3O8-　each　Nb　atom　reaches　its　maximum　oxidation　state　of　+5.　The　structures　and　electronic　states　for　the　triniobium　oxide　clusters　are　significantly　different　from　the　corresponding　tritantalum　oxide　clusters,　in　particular,　for　Nb3O3-,　Nb3O5-,　and　Nb3O7-.　Molecular　orbital　analyses　are　performed　to　elucidate　the　chemical　bonding　and　the　electronic　and　structural　evolution　in　these　triniobium　oxide　clusters.