ABSTRACTHeparin‐doped polypyrrole (PPy‐Hep) and ‐doped polypyrrole (PPy‐ClO4) films are synthesized onto FTO‐coated glass electrode in a potentiostatic electrochemical process with the aim of producing uniform, transparent, and adherent coating. The resultant polymers are characterized via cyclic voltammetry, scanning electron microscopy (SEM), and UV–vis absorption spectroscopy. SEM study indicates that the PPy‐Hep film to be composed of a continuous interlinked network of quasi spherical grains (50–80 nm in dimensions). The electrochromic properties of PPy‐Hep and PPy‐ClO4 polymer films are compared to spectroelectrochemistry and switching studies. The effect of different solvents (water, propylene carbonate, and acetonitrile) on the electrochromic features of electropolymerized polymers has been investigated, and we find a very significant solvent effect. PPy‐Hep film exhibits switching time of 1 s and the maximum transmittance contrast (ΔT%) is 48% at 800 nm in water. In addition, presence of Hep causes drastic enhancement of electro‐optical stability of PPy. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3365–3371