Time-dependent 17O NMR spectra of basified decaniobate (Nb10O286–) solutions displayed intense resonances assigned to the well-known protonated hexaniobate anion (Nb6O198–) and two other species identified as heptaniobate (Nb7O229–) and protonated tetracosaniobate (Nb24O7224–) anions. The decaniobate ion showed no sign of protonation from pH 6–10, in contrast with the hexaniobate ion which was protonated at doubly-bridging oxygen sites when the pH was 10–13. Most (> 90 %) of the heptaniobate formed 1 h after basification was transformed into other species after 3 weeks. Tetracosaniobate was formed reversibly from decaniobate, but only when KOH, NaOH and [(CH3)4N]OH were employed; none was observed after basification with [(n-C4H9)4N]OH. Moreover, far more tetracosaniobate was formed from KOH than from [(CH3)4N]OH. This effect was attributed to a tetracosaniobate cation binding site that binds K+ more readily than (CH3)4N+ but is too small to accommodate (n-C4H9)4N+.