A series of polyoxometalates (POM)/1,10-decanediamine ultrathin films were fabricated by self-assembly, and their photochromic properties were investigated. It is found that the films fabricated from molybdenum POMs show better photochromic responses than those from tungsten POMs. The photochromic responses of these hybrid films are related to the reduction potentials of the POMs. The higher the reduction potential of the POM is, the better the photochromic response of the hybrid film. Among these systems, the PMo12O40/ 1,10-decanediamine ultrathin film with a thickness of 70 nm shows a change as high as 0.168 in UV-visible absorption spectra before and after 10 min of UV irradiation.