Effects of isolobal heteroatoms in divanadium-substituted gamma-Keggin-type polyoxometalates, (TBA)(4)[gamma-XV2W10O38(mu-OH)(2)] 1(X) and (TBA)(4)[gamma-XV2W10O38(mu-O)] 2(X) (where X = Ge or Si), on (OV)(2)(mu-OH)(2) and (OV)(2)(mu-O) core structures and transformations from 2(X) to 1(X) have been investigated. X-ray crystallography of 1(X) and 2(X) reveals that larger Ge (covalent radius 1.22 angstrom; covalent radius of Si 1.11 angstrom) induces (a) expansion of (OV)(2)(mu-OH)(2) and (OV)(2)(mu-O) cores, (b) expansion of lacunary sites, and (c) deep location of divanadium cores inside their lacunary sites. Density functional theory (DFT) calculations for anionic moieties of 1(X) and 2(X) reveal that energy levels of the highest occupied molecular orbital (HOMO)-1 in 1(Ge) and HOMO in 2(Ge) are lower than those in 1(Si) and 2(Si), respectively, because of smaller contribution of p(z) orbitals of oxygen atoms in 1(Ge) and 2(Ge), which would result from shorter V center dot center dot center dot O(-Ge) distances. Compound 2(Ge) reacts with water vapor to form (TBA)(4)[gamma-GeV2W10O38(mu-OH)(2)] 1'(Ge) via a crystal-to-crystal transformation, and the water dissociation proceeds heterolytically. DFT calculations reveal that the reaction proceeds through (1) coordination of water on a coordinatively unsaturated site of vanadium in the lowest unoccupied molecular orbital (LUMO), followed by (2) proton transfer to the bridging oxo moiety. The order is different from that in 2(Si), which would result from the lower energy level of HOMO of 2(Ge) (i.e., lower nucleophilicity toward a proton of water) than that of 2(Si).