Wholly aromatic, main-chain copolyesters derived from terephthalic acid and phenylhydroquinone modified with m-hydroxybenzoic acid were prepared and characterized by elemental analyses, C-13 nuclear magnetic resonance, thermogravimetric analysis, optical observations, differential scanning calorimetry and wide-angle X-ray diffraction. The phases behaviours depended on copolymer composition. Poly(p-phenylphenylene terephthalate-co-m-hydroxybenzoate)s with molar percentages of the p-phenylphenylene terephthalate units in excess 70 mol% displayed thermotropic mesophases at relatively low temperatures and exhibited good thermal stability up to about 400 degrees C. Increasing content of m-hydroxybenzoic acid units, which increased the amount of structural irregularity, decreased the isotropization temperature and the temperature at which the melt started to flow, and also caused a loss of both crystallinity and of liquid crystallinity.