Abstract: The diffusion of water into polycarbonate film was investigated by attenuated-total-reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Via band deconvolution in the O-H bending region, three components were identified, which were located at 1669, 1648 and 1623 cm^-1, respectively, whereas in the O-H stretching region, three bands at 3562, 3423 and 3246 cm^-1, respectively were isolated. These results indicate the presence of three states of water molecules in the polycarbonate matrix:strong, moderate, and weak hydrogen-bonded water. Two dimensional correlation analysis of the IR data further confirmed the presence of the three states, and revealed the sequence of the formation of the three types of hydrogen bonding. Based on these experimental results, we concluded that when water diffuses into polycarbonate matrix, it first forms moderate hydrogen bond with the carbonyl group(i. e. the water bound to the carbonyl), followed by the formation of strong hydrogen bonds due to a water molecule bridging a carbonyl and another carbonyl or water molecule, and weak hydrogen bond between water molecules in microvoids.