Recently reported studies have shown some optical limitations of dual electrochromic systems. Among all the possible combinations between two materials in a dual system, the maximum contrast available was lower than that obtainable by one of the constituent materials by itself. In this study. we mathematically prove that this is a valid result for any electrochromic system constituted by materials that obey the Beer-Lambert law. (C) 2009 Elsevier B.V. All rights reserved.