Hydration behavior of phospholipid Langmuir-Blodgett (LB) films in water was studied by using a quartz-crystal microbalance (QCM) as a substrate. The hydration rate and amount were obtained from time-courses of frequency changes of the QCM deposited with LB films of various phospholipids such as phosphatidylethanolamine (DLPE, DMPE, and DPPE), phosphatidylcholine (DPPC), phosphatidylglycerol (DPPG), phosphatidylserine (DPPS), and cholesterol. The phosphatidylethanolamino (PE) LB films showed the large hydration (water penetration) only around their phase transition temperatures (T(c)), in which two domains of crystal and fluid liquid-crystalline states coexist. The acyl chain length affected the hydration rate but not the hydration amount. The LB film of DPPC and DPPG having relatively hydrophilic head groups existed as a stable membrane in the solid state below their T(c); however, it gradually flaked from the substrate in the fluid state above their T(c). On the other hand, the LB film of DPPS having relatively water-unacceptable head groups hardly hydrated at any temperatures both below and above T(c). In the case of cholesterol LB films, water can penetrate into the structure defects of the membrane at low temperature, but water is removed at high temperature due to the aging effect.