Electrochromic ultrathin films composed of the polyoxometalate cluster K(10)[P(2)W(17)O(61)]center dot 17H(2)O (P(2)W(17)), polyallylamine hydrochloride (PAH) and Neutral Red (NR) were fabricated on quartz, silicon wafer and indium-tin oxide substrates by the layer-by-layer self-assembly method. The multilayer [P(2)W(17)/PAH/P(2)W(17)/NR](n) films were characterized by UV-vis spectroscopy, scanning electron microscopy, cyclic voltammetry and chronoamperometry and in-situ spectral electrochemical measurements, which exhibited suitable response time, high optical contrast and low operation potential. The monolacunary polyoxometalate-based electrochromic film displays adjustable colors and undergoes transitions from deep pink to light purple, then to dark purple-blue over the potential range from +0.2 V to -1.0 V. Furthermore, two types of data storage using optical changes of the [P(2)W(17)/PAH/P(2)W(17)/NR](35) film are proposed. The results provide valuable information for exploring applications in tunable-color electrochromic devices and data-storage devices. (C) 2009 Elsevier B.V. All rights reserved.