Electrochromic devices that respond to varied levels of applied electrical potential by changing light opacity are presently prepared from either insertion compounds or reversible electrodeposition of metals from solution. We report here another approach to electrochromics based on the reversible deposition/dissolution of silver oxide onto FTO-coated glass from an aqueous solution containing silver (I)-ammonia complexes. The basic redox chemistry underlying the functioning of this electrochromic system has been identified using traditional techniques such as X-ray diffraction, scanning electron microscopy, and cyclic voltammetry, as well as a quartz nanobalance for measuring the working electrode weight at different potentials. The AgNH3+ <-> AgO reaction is responsible for the coloration/decoloration of the working electrode, which can be repeatedly cycled between various states of visual opacity. Optical measurements reveal that the transmittance of the working electrode in the visible region drastically drops from 80% without silver oxide deposition to 4% when it is coated with a silver oxide him. (C) 2000 Elsevier Science B.V. All rights reserved.