Electrochromic materials are being studied for applications involving infrared emissivity modulation. The materials being investigated are amorphous WO3 and poly-crystalline WO3, NiO, and Ta2O5. Hydrogen ions are intercalated into and deintercalated from the films leading to changes in the optical properties of the materials. Visible and infrared ellipsometry are used to measure the optical constants of these materials in various states of ion intercalation and to determine the reversibility of the reactions. The spectral range for the optical constants determination is from 0.3 to 14 mu m. Simulations of electrochromic device structures using these optical constants show an emissivity modulation greater than 50% for wavelengths near the peak of the room temperature blackbody curve.