We have found that pi-A isotherms of amphiphilic compounds forming condensed monolayers such as long-chain acids, long-chain esters, and phospholipids, expanded a lot by thermal treatment of monolayers on the water surface up to certain high temperatures. These phenomena were explained by incorporation of water molecules among the hydrophilic groups in the monolayers by the thermal treatment based on the experimental results of the quartz crystal microbalance method. It took rather long time to incorporate water molecules even at higher temperatures, but the incorporated water molecules could be easily pulled out in a short time from the Langmuir-Blodgett films in drying atmosphere even at room temperature. From these results, we can conclude that water should be an essential component of monolayers at the water surface when they are equilibrated with water.