Rates of steady oxygen-isotope exchange differ in interesting ways for two sets of structural oxygens in the [HxTa6O19]((8-x)-)(aq) Lindqvist ion when compared to published data on the [HxNb6O19](8-x)(aq) version. Because of the lanthanide contraction, the [HTa6019](8-x)-(aq) and [HxNb6Oj9](8-x)-(aq) ions are virtually isostructural and differ primarily in a full core (Kr vs Xe) and the 04 electrons in the [HxTa60l 9](8- x)-(aq) ion. For both molecules, both pH-dependent and -independent pathways are evident in isotopic exchange of the 12 112-0(H) and 6 q=O sites. Rate parameters for y=O exchange at conditions where there is no pH dependence are, for the Ta(V) and Nb(V) versions respectively, Wo" = 2.72 x 10-5 s-1 and 9.7 x 10-1 s-1, AW = 83.6 3.2 and 89.4 kJ-mol-1, and As = -51.0 10.6 and -42.9 J-mol-l-K-1. For theU2-0 sites, ko.. = 1.23 x 10-6 s-1, AW = 70.3 9.7 and 88.0 U-mol-', and AS* = -116.1 32.7 and -29.4 J-mol-l-K-1. Protonation of the 6 77=0 sites is energetically unfavored relative to the 12 U2-0 bridges in both molecules, although not equally so. Experimentally, protonation labilizes both the Y2-0(H) and 77=0 sites to isotopic exchange in both molecules. Dens ity-f u nctio nal electronicstructure calculations indicate that proton affinities of structural oxygens in the two molecules differ with the [H,Ta60,](8-x)- (aq) anion having a smaller affinity to protonate than the [H,Nb60l9]8-x(aq) ion. This difference in proton affinities is evident in the solution chemistry as pKa = 11.5 for the [HTa60l9]'-(aq) ion and pKa = 13.6 for the [HNb6Ol9]'-(aq) ion. Most striking is the observation that q=O sites isotopically equilibrate faster than the Y2-0 sites for the [HxTa6019](1-x)-(aq) Lindqvist ion but slower for the [HxNb60l 9](8-x)-(aq) ion, indicating that predictions about site reactivities in complicated structures, such as the interface of aqueous solutions and oxide solids, should be approached with great caution.