The use of highly conducting polyaniline films as active layers in electrochromic devices operating in the mid-infrared, IR, range (8-12 mu m) has been investigated. Upon reversible electrochemical doping, the specular reflectance of thin films deposited on an IR transparent window and electrode switches from 0.2 to 0.65 around 12 mu m. The complex refractive index of the conducting polymer has been determined by ellipsometry in the range 0.3 to 16 mu m. Optical calculations of the multilayer structure demonstrate that the active layer thickness to maximum contrast should be close to 300 nm. Devices incorporating a lithium or protonic gel ionic conductor and WO3 as ion storage layer have been built. Optical performance in the infrared is similar to that of the layer in contact with a liquid electrolyte. The response time (t(90)) determined from optical density changes at 633 nm is of a few seconds. Moreover, devices incorporating the Li+ conductor retained 81% of the original reflectance contrast after 900 cycles.