The electrochemical and optical properties of poly(isothianaphthene) (PITN) during electrochemical doping have been investigated by cyclic voltammetry and-optical absorption spectrum measurements. PITN can be reversibly cation- and anion-doped without decomposition of the material. This polymer, PITN, with these two reversible and stable redox states of different colors is a potential candidate for electrochromic displays. The existence of a residual charge (positive or negative) depending on the electrochemical cycle is pointed out a conditions to obtain the more or less full neutralization of this residual charge are described. Further, a reading-writing device using a residual charge which remains PITN after the dedoping has been proposed.