Knowledge of dissolved H2O in primary Kilauean magmas provides a constraint on H2O abundance in the deep mantle region that feeds the upwelling plume beneath Hawaii. Given an H2O/K2O mass ratio of ~1.3 for basaltic glasses and melt inclusions from Kilauea, the mantle source is estimated to contain 450¿190 ppm H2O. This value is ~3 times greater than that estimated for the mantle source for mid-ocean ridge basalts. Consideration of OH solubility in olivine suggests that water undersaturated melting of the upwelling Hawaiian plume probably begins at a depth of ~250 km. Thus during plume ascent through most of the upper mantle, water is partitioned between nominally anhydrous silicates and a small mass fraction of entrained hydrous silicate melt. Because water strongly influences the viscosity of olivine aggregates, partial melting and melt extraction will have important effects on the rheology of the upwelling plume beneath Hawaii. ¿ 1998 American Geophysical Union