The penetration process of water into a stretched cellulose acetate (CA) film was investigated in real time by coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The red shift of the C=O stretch mode due to hydrogen-bond formation was smaller than that of the unstretched film. The appearance of two Raman bands at 1605 and 1665 cm(-1), which was instantaneously observed for the unstretched CA film, was reproduced but with much slower temporal response. The present results indicate significant effects of the stretching procedure on the water penetration process of CA films.