An ultrathin multilayer film consisting of polyoxometalate cluster alpha-K-8[P2W17O61NiII(H2O)](.)18H(2)O(P2W17Ni) and polyallylamine hydrochloride (PAH) was fabricated on quartz and indium tin oxide substrates by layer-by-layer self-assembly. The multilayer films, (P2W17Ni/PAH)(n), were characterized by UV-visible spectra, cyclic voltammetry, chronoamperometry, atomic force microscopy, and in situ spectroelectrochemical measurements. The multilayer film exhibits both suitable response time and low operation potential due to the presence of nickel-substituted polyoxometalate, providing valuable information for exploring the possible application of polyoxometalate-based electrochromic materials. Further, P2W17Ni embedded in a self-assembled polyelectrolyte matrix can also be used as a pH sensor because of the pH dependence of its electrochemical response. (c) 2005 The Electrochemical Society. All rights reserved.