New electrochromic copolymers of 3,4-ethylenedioxythiophene and di(m-carboranyl thiophene) were synthesized, characterized and their electro-optical properties were reported. The copolymers were successfully prepared from various monomer feed ratios using electrochemical polymerization technique. It can be conjectured that electrochemical and optical properties of the copolymers can be adjusted by playing with monomer feed ratios. For example, the maximum wavelength of poly(3,4-ethylenedioxythiophene) at 600 nm can be shifted to lower values (i.e., 522 nm) by increasing the ratio of di(m-carboranyl thiophene) in the monomer mixture. The copolymer films also have low band gap values between 1.69 and 1.82 eV and showed electrochromic properties; purple when neutralized and transparent sky blue when oxidized. Also, during redox switching the films exhibited a percent transmittance change between 32% and 46% with a switching time between 1.0 s and 1.3 s. (C) 2013 Elsevier B.V. All rights reserved.