The phase behavior of mica-supported bilayers has been studied with a differential scanning calorimeter, owing to a significant increase of the substrate area resulting from chopping a piece of mica into a large number of microscopic chips. We have found that then are two main phase transitions for supported phosphatidylcholine bilayers, separated by about 2 degrees C, and that the lower transition temperature is about 2 degrees C higher than the main phase transition temperature of lamellar bilayers. Therefore, supported bilayers do have the essential feature of free-standing lipid bilayers. However, the double transitions and higher transition temperatures indicate that details of the main phase transition in lipid layers are highly sensitive to their surrounding environment.