The synthesis and electrochemical polymerization of 3,3-diethyl-3,4-dihyro-2H-thieno-[3,4-b][1,4]dioxepine (ProDOT-Et) was performed resulting in a stable electrochromic polymer capable of switching between an absorbing blue neutral state and a highly transmissive sky blue oxidized state in sub-second time frames. High optical switching contrast ratios (up to 75% at lambda(max)) and high composite coloration effiencies (505 cm(2)/C) were measured.