A series of photochromic hybrid films were prepared through entrapping tungsten heteropolyoxometallate (PW12O403-) and molybdenum heteropolyoxometallate (PMO12O403-) in polyacrylamide (PAM) matrix, the corresponding structure and photochromic behaviors of the films were investigated with Fourier transform infrared (FT-IR), UV-Vis-near-infrared (NIR) absorption spectra (UV-Vis-NIR) and electron spin resonance (ESR). FT-IR results showed that the Keggin geometry of heteropolyoxometallate was still preserved inside the composites and strong Coulombic interaction was built between heteropolyoxometallate and PAM via hydrogen bonding. Irradiated with ultraviolet light, the transparent films changed from colorless to blue and showed reversible photochromism. The bleaching process occurred when the films were in contact with air or O-2 in the dark. The molybdenum heteropolyoxometallate hybrid film had higher photochromic efficiency and slower bleaching reaction than tungsten heteropolyoxometallate hybrid film. The characteristic signals of radical and W(V) or Mo(V) in ESR spectra indicated that PAM is a hydrogen donor and the photoreduced process is in accordance with the radical mechanism. (C) 2002 Elsevier Science B.V. All rights reserved.