We compare oxygen-isotope exchange rates for all structural oxygens in three polyoxoniobate ions that differ by systematic metal substitutions of Ti(IV) f Nb(V). The [HxNb10O28](6-x)-, [HxTiNb9O28](7-x)-, and [HxTi2Nb8O28](8-x)- ions are all isostructural yet have different r??nsted properties. Rates for sites within a particular molecule in the series differ by at least a??104, but the relative reactivities of the oxygen sites rank in nearly the same relative order for all ions in the series.