A new soft chemical route to [Ta₆O₁₉]⁸⁻ has been developed by the dissolution of [Ta(O₂)₄]³⁻ in conditions alkaline enough to arrest formation of Ta₂O₅, followed by [VO₄]³⁻-catalyzed decomposition of the peroxide ligands and crystallization of the salt. An average of bond lengths and angles from isostructural salts of [Ta₆O₁₉]⁸⁻ and [Nb₆O₁₉]⁸⁻ indicate there is an increase in terminal M(ηO) bond lengths and M-µ₂-O–M angles and a decrease in bridging µ₂-O–M bond lengths in [Ta₆O₁₉]⁸⁻, although the central µ₆-O–M bond lengths are identical within experimental error. Two new structures of Na₇[HNb₆O₁₉]·15H₂O (1) and Na₈[Ta₆O₁₉]·15H₂O (2) are exemplary of the fact that protonated µ₂-OH are observed exclusively in the niobates. In these structures, the metal-oxide framework, seven sodium atoms, and all fifteen water molecules are located in identical unit cell positions, but in 2 an eighth charge-balancing sodium is located in close proximity to the protonated µ₂-OH in 1. Differences in the basicity of Nb(V)- and Ta(V)-bound oxygen atoms are also manifested at the surfaces of ¹⁷O-enriched powders of Nb₂O₅ and Ta₂O₅. Oxygen exchange at the surface of these materials readily takes place at both terminal and bridging sites in Nb₂O₅ but only at terminal sites in Ta₂O₅.