A novel kind of electrochemically doped poly(5-vinyl-2,2’:5’,2"-terthiophene) has been prepared by electrochemical doping of poly(5-vinyl-2,2’:5’,2"-terthiophene) and by electrolytic polymerization of 5-vinyl-2,2’:5’,2"-terthiophene in the presence of tetra-n-butylammonium perchlorate as a supporting electrolyte. The electrochemically doped poly(5-vinyl-2,2’:5’,2"-terthiophene), obtained as bluish purple lustrous films, is identified as a partially oxidized radical-cation salt with ClO4- as a dopant, having a partially cross-linked structure due to the coupling reaction of the radical cation of the pendant terthiophene moiety. The polymer with a degree of doping of 45% exhibits a room-temperature conductivity of 2 x 10(-8) S cm(-1) with an activation energy of 0.5 eV. The polymer undergoes a reversible clear color change from bluish purple to pale yellowish orange and vice versa on electrochemical dedoping and doping and functions as a novel class of potential electrochromic material.