A variable transmission window comprises outer panes of glass 17 and 19 and a multi layer electrochromic device allowing variation in the optical transmission properties of the window. The device comprises a transparent electrically conductive layer 12 such as indium tin oxide, an electrochromic material 13 such as a transition metal oxide bronze, a solid electrolyte 14 which is a fast ion conductor of ions of the metal which dissolves in the electrochromic material, a second electrochromic material 15, and a second transparent electrically conductive layer 16. The electrochromic material may be MoO.sub.3 or WO.sub.3. The solid electrolyte 14 may be a Bordeaux glass of LiCl, Li.sub.2 O, B.sub.2 O.sub.3. A quantity of metal colouration atoms, for example lithium, may be transferred reversibly between the electrochromic layers 13 and 15 by applying potentials to contacts 21 and 22. A given quantity of metal coloration atoms produces a coloration of greater density in one electrochromic layer than in the other, for example by having one layer 15 thicker than the other layer 13.