Conjugated polymers exhibit large changes in their optical and electronic properties upon either n- or p- type doping. We report details of the synthesis of poly(2,5-dimethoxyphenylene vinylene)s (DMPPV)s, prepared from sulphonium precursor polymers with a range of counter-anions, and optical measurements of their electronic transitions and infrared active vibrations. We have observed that the DMPPV produced when the sulphonium polyelectrolyte precursor polymer contained SbF6- as the counterion shows sub band-gap optical absorption bands. This was observed to take place during thermal conversion from the precursor to the conjugated polymer under acidic conditions. © 1991.