The photochromic process of nanocomposite film containing nanometer-sized Keggin type phosphotungstic acid uniformly dispersed in polyacrylamide matrix was studied by in situ AFM and spectroscopy. Under UV irradiation, the film was reduced photochemically to yield blue species. With the increase of irradiation time, the photoreduction proceeded from one-electron to two-electron blue stage. During the photochromism, the size and morphology of composite particles changed due to the cross-linked reactions of polymeric radicals. The XPS studies indicated that the chemical environment of tungsten changed and the W5(+)/W ratio increased as irradiation time prolonged. FT-IR results showed that the Keggin geometry of phosphotungstic acid was still preserved inside the composite during photochromism, and the interactions between polyanions and polymer matrix increased with the irradiation time. It was suggested that the degree of the photoreduction process depended on the varying of interactions between phosphotungstic acid and polymer. (c) 2005 Elsevier B.V. All rights reserved.