The new polymer poly(thiophene-2,5-diyl-1,2-dimethoxyvinylene) was synthesized by a benzoin condensation of thiophene-2,5-dicarboxaldehyde to poly(2,5-thienylene-1-oxo-2-hydroxyethylene) (M(w) similar to 1 x 10(6), pd similar to 13), followed by O-alkylation using dimethyl sulfate. A side reaction of the benzoin condensation step leading to dimethyl-2,5-thiophenedicarboxylate can be avoided by using cyclohexanol as the solvent. The O-alkylation, including the workup, has to be performed under nitrogen protection to counteract the degradation of the polymer by air under the influence of light. A mechanism is proposed for the oxidative degradation of this poly(thiophene-2,5-diylvinylene) derivative and other poly(arylenevinylenes). Conductivity and thermal properties of the new polymer are given.