A series of nanocomposite hybrid films consisting of phosphotungstic acid (PW12) and polyacrylamide (PAM) were prepared. TEM images showed that PW12 particles with average diameter of tens of nanometer were finely dispersed in the film. FT-IR results suggested that the Keggin geometry of PW12 was still preserved inside the matrix and a charge-transfer bridge was built between PW12 and PAM through hydrogen bonds. Under UV irradiation, PW12 was reduced photochemically to yield various mixed-valency colored species ("heteropoly blues" or "heteropoly browns"). The extent of photoreduction was concerned with the concentration of PW12 and irradiation time. The presence and diffusion of oxygen played an important role in reversibility of photochromism. (C) 2003 Kluwer Academic Publishers.