Copolymerizations at various feed ratios between azo-dye-substituted thiophenes and 3-hexylthiophene under FeCl3-based oxidative conditions have been conducted. It is shown that when the molecular structure of the dye is properly designed, completely soluble copolymers, with molecular weigths ranging around 10(4) as M-n and dye-substituted monomer content up to 50% in a molar ratio, may be prepared, as well as a completely soluble homopolymer. In addition, the so-obtained polymers evidenced excellent filmability from a variety of solvents. The dye-substituted monomer content of the soluble copolymers has been assessed by H-1 NMR analysis via a convenient choice of the reference signals. In the completely soluble copolymers the dye-substituted monomer content is well correlated to the feed within a range between 0 and 75% of dye-substituted unit in the feed, which corresponds to a 0-50% range in the copolymer composition, still conserving satisfactory molecular weights. Following these observations and analyzing the characteristics of the polymers, a lower reactivity of the dye-functionalized unit toward 3-hexylthiophene was evidenced in the used reaction conditions, and some hypotheses on this finding are presented.