We report a femtosecond time-resolved study of ultrafast vibrational dynamics of membrane-bound water. The O-H stretch vibrational lifetime of water bound to a phosphatidylserine lipid monolayer is measured by employing a pump-probe scheme and using vibrational sum frequency generation (VSFG) as a surface-specific probe. We conclusively demonstrate that water molecules adjacent to a lipid layer are energetically decoupled from the bulk and therefore constitute an intrinsic part of the membrane. This contrasts starkly with recent observations by us and other groups that other types of interfacial water exchange energy efficiently with the underlying bulk.