We studied flexible electrochromic devices containing phthalate derivative which showed reversible color change between water transparent and subtractive primary color by electrochemical reaction. Poly(vinyl butyral) (PVB)-based gel electrolyte was prepared to apply the flexible electrochromic device. The ionic conductivity of the gel electrolyte depended on the polymer content, and was higher than 10(-4) S/cm at 25 degrees C at the PVB content of 33 wt%. Redox reaction of phthalate derivatives was successfully achieved in the gel electrolyte. It is revealed that PVB-based gel electrolyte works well as the material for flexible electrochromic devices showing subtractive primary colors. (c) 2007 Elsevier Ltd. All rights reserved.