The constitution of poly(4-oxybenzoate-co-1,4-phenylene isophthalate)s, consisting of rod-like 4-oxybenzoyl and 1,4-phenylenedioxy units and angular isophthaloyl units, is modified by gradually replacing (i) the 1,4-phenylenedioxy units with chloro-1,4-phenylenedioxy units, and (ii) the angular isophthaloyl units with rod-like terephthaloyl units. The former modification results in a gradual decrease of the melting temperature, while the latter one has the opposite effect. When both modifications are employed simultaneously, their effects essentially compensate each other. Thus, a fully aromatic nematogenic copolyester consisting of equimolar quantities of rod-like 4-oxybenzoyl, chloro-1,4-phenylenedioxy and terephthaloyl units melts at 313 degrees C, more than 100 degrees C below its thermal stability limit. Although the melting behaviour of the materials investigated is strongly affected by annealing, the changes can be reversed by remelting.