The electrochromic cermet Au‐WO3is composed of grains of Au, approximately 20–120 A˚ in diameter, embedded in a matrix of amorphous WO3. The optical properties of the cermet, in its red electrochemically colored state, are different from the optical properties of its two components. WO3becomes highly electrically conducting when colored, but colored WO3in the presence of Au grains remains an insulator. A study of the optical and electrical properties of the cermet as a function of gold concentration is presented.