Quartz-crystal microbalance (QCM) and Fourier transform infrared (FTIR) reflection-absorption (RA) techniques are employed to investigate the binding mechanism of fully hydrated (thermally treated) DPPC Langmuir (L) films with sucrose in the aqueous solution. Sucrose molecules incorporated in the fully hydrated L film strongly bind to the L film even when the L film is laterally compressed to a high surface pressure, while those in partially hydrated (without thermal treatment) L films are squeezed out by the compression. When the L film is compressed to a low surface pressure corresponding to the liquid-crystalline (soft) state, the incorporated sucrose molecules remain well in both fully and partially hydrated L films. The amount of bound sucrose in the fully hydrated soft L film linearly increases with the concentration of sucrose. This indicates that the fully hydrated soft DPPC L film forms a mingled layer where a hydrated DPPC film and deeply incorporated sucrose molecules are inseparable from each other. In other words, incorporated sucrose molecules in the surface of the fully hydrated soft DPPC L films are very stable.