3-n-dodecylthiophene-3-methylthiophene copolymers are novel compounds having repeating units represented by the formula, ##STR1## and repeating units represented by the formula, ##STR2## randomly and linearly arranged and possessing a weight average molecular weight in the range of 60,000 to 100,000. 3-n-dodecylthiophene-3-methylthiophene copolymer compositions are novel compounds obtained by doping the copolymers mentioned above with a dopant. The copolymer compositions are obtained by electrolyzing a solution which consists of a supporting electrolyte capable of releasing a dopant, 3-n-dodecylthiophene, 3-methylthiphene, and a solvent. The copolymers mentioned above is produced by reducing the product of the electrolysis.