Differential scanning calorimetry and wide-angle X-ray diffraction techniques are employed to investigate the melting behaviour and crystalline structure of poly(1,4-phenylene isophthalate) and poly(4-oxybenzoate-co-1,4-phenylene isophthalate) containing equimolar quantities of the constituent 4-oxybenzoyl, 1,4-phenylenedioxy and isophthaloyl units. These materials can contain two different crystalline structures, both consisting of 1,4-phenylene isophthalate sequences. The formation of these polymorphs in both the homopolymer and copolymer is influenced by the crystallization and/or annealing conditions. The melting behaviour of the copolymer is strongly influenced by annealing, but these changes can be reversed by remelting. Consequently, they are not caused by changes in chain constitution arising from transesterification.