WO3∙H2O powder embedded in a plastic matrix has demonstrated attractive properties as a flexible emissivity modulator driven by the electrochemical insertion of lithium. The near infrared optical properties of LixWO3∙H2O are investigated as a function of the lithium intercalation level x for 0≤x≤2.65. The reflectivity at 2.5 μm, measured through a glass window, was found to vary between 0.15 and 0.4 over the α intercalation phase (0≤x≪0.3), i.e., demonstrating an emissivity contrast of ∼0.25. For x≥0.1 the LixWO3∙H2O-based plastic device exhibits a Drude behavior comparable to that of monoclinic tungsten oxide (m-WO3) thin films. For larger intercalation levels, the performances are affected by some irreversible structural modifications of the LixWO3∙H2O lattice. © 2004 American Institute of Physics.