The penetration of water into a cellulose acetate (CA) film with m-terphenyl (m-TP) as a waterproofing agent (WPA) was investigated in real time by coherent anti-Stokes Raman scattering (CARS) microspectroscopy. Owing to the WPA, the water penetration was significantly suppressed. It was manifested by a small spectral change in comparison with the CA films without m-TP. Moreover, the amplitude of the Raman band at 1600 cm(-1) due to the OH bending mode, which is the marker band of water penetration, was significantly smaller. The present results indicate that m-TP has significant effects as a WPA for CA films on the water penetration process.