The bound water associated with phosphatidylethanolamine (PtdEtn) in the lamellar and inverted hexagonal structures is determined directly. Bound water is considered as that water which is unavailable for solvation of the polar solute sucrose. In the fluid lamellar (L(alpha)) state of dioleoyl PtdEtn (at 2 degrees C), 7.2 water molecules per phospholipid are bound and unavailable as a solvent for sucrose. In the inverted hexagonal structure (16 degrees C and 30 degrees C), 5.4 and 5.6 water molecules per phospholipid, respectively, are unavailable for solvation. Similar results are obtained for egg PtdEtn (L alpha, 15 degrees C, 7.5 water per Lipid; L alpha, 28 degrees C 6.9; H-Pi, 40 degrees C, 5.1). Weakly binding polar solutes(glycine and acetate) yield comparable trends that support a dehydration at the lamellar to inverted hexagonal phase transition of approximately 2 water molecules per PtdEtn. This is the first direct determination of the changes in hydration that occur in the lamellar to inverted hexagonal transition.