The electrochromic performance of poly [2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diylalt-thiophene-2,5-diyl] (TQ1), switching from an intense blue color to a transmissive yellow-brown oxidized state, are presented. Additionally, two new polymers, based on the same polymeric backbone but with oligoethylene oxide (TQ2) and ethylhexyloxy (TQ3) side chains, were prepared to compare the structure-property relationships. TQ1 has the highest optical contrast, with a transmittance change of 50%, recorded by spectroelectrochemistry at the low-energy absorption maximum (623 nm). The high optical contrast, facile synthetic preparation, and processability through good solubility in organic solvents make TQ1 an interesting polymer for implementation in non-emissive electrochromic devices. Response times were improved by the introduction of ethylhexyloxy side chains (TQ3), where 92% of the transmittance modulation was maintained at switching times of 0.5 s. The influence of film thickness on optical contrast was also investigated, where the highest optical contrast for TQ1 was obtained for films having reduced absorption of 0.84 a.u. at the low-energy absorption maximum. All three polymers show decent electrochemical stability over 200 full switches.