A thermogravimetric technique has been used to obtain accurate measurements of water vapor dissolution in molten CaO-MgO-SiO2 slags. These measurements indicate that the solubility of water vapor is independent of temperature over a 200 °C range (1375 to 1575 °C) and varies directly with the square root of the water vapor partial pressure. Water vapor was observed to be amphoteric in nature with respect to dissolution in liquid silicates. A minimum in solubility was observed at approximately the metasilicate composition. Results obtained during the present study indicate that the empirically defined basicity index(N Cao + NMgo)/Nsio2 is inadequate to describe the dissolution of water vapor in silicate melts. The activity of silica has been identified as a more accurate indicator. Based on this approach, ambiguous results from previous studies in the CaO-MgO-SiO2 slag system have been rationalized. From a statistical evaluation of the data, a relationship in terms of the activity of silica has been obtained which permits calculation of water vapor contents in CaO-MgO-SiO2 slags.