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The solubility of water vapor at 750° to 1050°C was determined for alkali borate melts containing 0 to 40 mole % Li2O, Na2O, or K2O. In all cases the solubility in these melts is linearly proportional to the square root of the H2O partial pressure. At pH2O = 1 atm and T= 900°C, o.5 to 2.2 mole % H2O are dissolved in the melts in equilibrium. In the potassium borate melts a minimum of solubility was observed at about 25 mole % K2O; in the sodium borate melts the minimum was at 35 to 40 mole % Na2O. In the lithium borate melts a minimum of solubility was not reached in the range of compositions investigated. These results are discussed in terms of a concept for the acid-base properties of melts and glasses in which the position of the solubility minimum corresponds to the “neutral point” of an acidity-basicity scale. Some corroborating qualitative observations concerning the evaporation of components from the glass melts and the chemical resistivity of the corresponding solid glasses are discussed.