The formation energetics of hydrous and dehydrated potassium hexaniobates are investigated using high-temperature oxide melt solution calorimetry. The enthalpies of formation of K4Nb6O17 and K(4)Nb(6)O(17)3H(2)O from oxides are (-864.42 +/- 10.63) and (-899.32 +/- 11.48) kJ/mol, respectively. The formation enthalpy of K4Nb6O17 from elements is (-7289.64 +/- 12.50) kJ/mol, and of K(4)Nb(6)O(17)3H(2)O is (-8181.94 +/- 13.24) kJ/mol. The enthalpy of dehydration (H-dehy) for the reaction K(4)Nb(6)O(17)3H(2)O (xl, 25 degrees C) = K4Nb6O17 (xl, 25 degrees C) + 3H(2)O (l, 25 degrees C) is endothermic and is 34.60 +/- 7.56 kJ/mol. The H-dehy per mole of water, 11.53 +/- 2.52 kJ/mol, indicates the water molecules in K(4)Nb(6)O(17)3H(2)O are not just physically adsorbed, but loosely bonded in the K4Nb6O17 phase, presumably in specific interlayer sites. The loss of this water near 100 degrees C on heating is consistent with the weak bonding of water.