Anisotropic homopolymerization of 2,5-dihydroxyterephthalic acid (DHTA) and 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT.2HCl), and copolymerization of DHTA, terephthalic acid and DABDT.2HCl in polyphosphoric acid (PPA), afforded high molecular weight rigid-rod polymer and copolymers, with two hydroxyl groups as pendants per repeat unit. In PPA, the lyotropic liquid crystalline behaviour of the homopolymer was distinctly affected by the polymerization temperature. Spectroscopic evidence strongly suggested a completely planar and ribbon-like structure, the consequence of the strong intramolecular hydrogen bonds between the OH moieties and the N atoms of the nearby benzothiazole units. The homopolymer also exhibited the unusual capability, characteristic of BBL-type ladder polymers, to form directly from its precipitate an aggregated film which displayed little or no shrinkage when dried. Anisotropic dopes of the homopolymer (dihydroxy-polybenzothiazole (PBZT)) and certain dihydroxy-PBZT/PBZT copolymers were spun into fibres with typical tensile properties for modified PBZT polymers, but marginal compressive strength (4-30 ksi (1 ksi = 6.9 M Pa) for homopolymer and 16 61 ksi for the copolymers). The intrinsic conductivity for the as-spun fibre of the homopolymer was 2.2 x 10(-7) S cm-1 and that for heat-treated (435-degrees-C) fibre was 5.6 x 10(-8) S cm-1.