Infrared-visible sum frequency generation spectroscopy (SFG) was used to directly probe water between polyurethane (PU) and sapphire substrates after exposing samples to liquid water and water vapor. For liquid water, the observation of SFG peaks associated with H2O bands (3000-3400 cm(-1)) and D2O bands (2300-2600 cm(-1)) indicated water molecules diffused to the buried interface and existed in the form of a hydrogen-bonded water network. The water layer disrupted interactions between polyurethane and sapphire. When PU films were exposed to water vapor, the SFG peak intensities of PU hydrocarbon and sapphire hydroxyl groups changed significantly, which suggested water molecules had reached the interface. However, no hydrogen-bonded water bands were present; instead, the H2O peak at 3550 cm(-1) and D2O peaks (2600-2700 cm(-1)) were observed. We assigned these peaks to low-coordination water molecules or hydroxyl groups hydrogen bonded with carboxyl groups of PU at the interface. The water molecules did not form a uniform layer at the interface and as a consequence did not completely disrupt the PU/sapphire interactions. These results provide important implications for understanding interfacial adhesion, coatings, and corrosion.