Structures of K-2(H2O)(2)B12F12 and K-2(H2O)(4)B12F12 were determined by X-ray diffraction. They contain [K(mu-H2O)(2)K](2+) and [(H2O)K(mu-H2O)(2)K(H2O)](2+) dimers, respectively, which interact with superweak B12F122- anions via multiple K center dot center dot center dot F(B) interactions and (O)H center dot center dot center dot F(B) hydrogen bonds (the dimers in K-2(H2O)(4)B12F12 are also linked by (O)H center dot center dot center dot O hydrogen bonds). DFT calculations show that both dimers are thermodynamically stabilized by the lattice of anions: the predicted Delta E values for the gas-phase dimerization of two K(H2O)(+) or K(H2O)(2)(+) cations into [K(mu-H2O)(2)K](2+) or [(H2O)K(mu-H2O)(2)K(H2O)](2+) are +232 and +205 kJ mol(-1), respectively. The calculations also predict that Delta E for the gas-phase reaction 2 K+ + 2 H2O -> [K(mu-H2O)(2)K](2+) is +81.0 kJ mol, whereas Delta H for the reversible reaction K2B12F12 ((s)) + 2 H2O(g) -> K-2(H2O)(2)B12F12(s) was found to be -111 kJ mol(-1) by differential scanning calorimetry. The K-2(H2O)(0,2,4)B12F12 system is unusual in how rapidly the three crystalline phases (the K2B12F12 structure was reported recently) are interconverted, two of them reversibly. Isothermal gravimetric and DSC measurements showed that the reaction K2B12F12(s) + 2 H2O(g) -> K-2(H2O)(2)B12F12(s) was complete in as little as 4 min at 25 degrees C when the sample was exposed to a stream of He or N-2 containing 21 Torr H2O(g). The endothermic reverse reaction required as little as 18 min when K-2(H2O)(2)B12F12 at 25 degrees C was exposed to a stream of dry He. The products of hydration and dehydration were shown to be crystalline K-2(H2O)(2)B12F12 and K2B12F12, respectively, by PXRD, and therefore these reactions are reconstructive solid-state reactions (there is also evidence that they may be single-crystal-to-single-crystal transformations when carried out very slowly). The hydration and dehydration reaction times were both particle-size dependent and carrier-gas flow rate dependent and continued to decrease up to the maximum carrier-gas flow rate of the TGA instrument that was used, demonstrating that the hydration and dehydration reactions were limited by the rate at which H2O(g) was delivered to or swept away from the microcrystal surfaces. Therefore, the rates of absorption and desorption of H2O from unit cells at the surface of the microcrystals, and the rate of diffusion of H2O across the moving K-2(H2O)(2)B12F12 ((s))/K2B12F12 ((s)) phase boundary, are evn faster than the fastest rates of change in sample mass due to hydration and dehydration that were measured. The exchange of 21 Torr H2O(g) with either D2O or (H2O)-O-18 in microcrystalline K-2(D2O)(2)B12F12 or K-2((H2O)-O-18)(2)B12F12 at 25 degrees C was also facile and required as little as 45 min to go to completion (H2O)((g)) replaced both types of isotopically labeled water at the same rate for a given starting sample of K2B12F12, demonstrating that water molecules were exchanging, not protons. Significant portions of mass (m) vs time (t) plots for the (H2O(g))-H-1,2/K-2((H2O)-H-2,1)(2)B12F12 ((s)) exchange reactions fit the equation m proportional to e(-kt), with 10(3)k = 1.9 s(-1) for one particle size distribution and 10(3)k = 0.50 s(-1) for another. Finally, K-2(H2O)(2)B12F12 was not transformed into K-2(H2O)(4)B12F12 after prolonged exposure to 21 Torr H2O(g) at 25 degrees C, 37 Torr H2O(g) at 35 degrees C, or 55 Torr H2O(g) at 45 degrees C.