A series of photochromic nanocomposite films were prepared by well-dispersed Keggin type polyoxometalates (POM) in polyvinyl alcohol (PVA). The corresponding structure, photochromic behaviors and mechanism of the films were investigated with transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared spectra (FT-IR), ultraviolet visible absorption spectra (UV-vis) and electron resonance spectra (ESR). In composite films, the sphere-shaped POM nanoparticles were well dispersed with narrow size distribution. AFM images indicated that the surface topography of polymeric matrix changed after adding POM, and the surface appearance of nanocomposite films was different before and after ultraviolet light irradiation. FT-IR results showed that the Keggin geometry of POM was still preserved inside the composites and a charge-transfer bridge was built between POM and PVA polymeric matrix through hydrogen bonding. Irradiated with ultraviolet light, composite films changed from colorless to blue and showed reversible photochromism. The composite film containing molybdenum exhibited faster photochromic efficiency and slower bleaching reaction than that containing tungsten. The inferior reversibility of composite films was observed during the coloration-decoloration cycle. The appearance of the characteristic signals of W5+ or Mo5+, in ESR spectra indicated that PVA was a proton donor and the photochemical process was in accordance with the charge-transfer mechanism. (c) 2007 Elsevier B.V. All rights reserved.