Electrochromic devices were investigated as a means of efficiently modulating the IR emissivity of space structures. The optical properties of electrochromic materials are modified upon application of a voltage allowing a high degree of control of the modulation. This degree of control is necessary for some low earth orbit space missions where effects such as rapid changes in the earth's albedo can change the temperature of a satellite. Optical constants of semicrystalline WO3 and NiO in their unintercalated and fully intercalated states were measured by variable angle spectroscopic ellipsometry over a spectral range of 2-14 mu m. Ta2O5 optical constants were measured for use as a hydrogen ion conductor and Au and ITO optical constants were measured for use as electrodes. Calculations were made of the emissivity modulation of an electrochromic device structure for a 300 K blackbody based on the measured optical constants.