The penetration process of water into a cellulose acetate film was traced in real time by coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The C=O stretch mode was red-shifted due to hydrogen-bond formation. We also found that two Raman bands at 1605 cm(-1) and 1665 cm(-1) emerged only in the early stage of the water penetration process. Based on the combined analysis of the experimental and computational studies, these bands at 1605 cm(-1) and 1665 cm(-1) were assigned as the OH bend mode due to hydrogen-bonded penetrated water and hydrogen-bonded OH groups in pyranose rings, respectively. (C) 2016 Elsevier B.V. All rights reserved.