Two series of semiflexible aromatic copolyesters have been prepared and studied. In the first series, the relative proportion of a biphenyl and a phenyl unit was varied through copolymerisation; in the second, the phenyl ring system was further substituted with an aliphatic side-chain, in order to promote additional disruption to any solid-state ordering. These copolymers have been characterised experimentally. The phase behaviour and morphology were closely inspected using a variety of techniques including wide angle X-ray diffraction, optical and scanning electron microscope and differential scanning calorimetry. The copolymers are proposed to be randomly copolymerised through a comparison of the results from X-ray diffraction with those from modelling. The modification of polymer repeat units alters the properties of the final polymers dramatically. Crystallisation of the copolymers is complicated. The rate of crystallisation is reduced through the introduction of side chains. The crystallisation behaviour is explained using a non-periodic lattice model by lateral matching of the similar units. Despite the low degree of polymerisation of the copolymers examined, the results do, nevertheless, point to a route whereby the properties of the polymeric systems in general may be controlled through molecular engineering. (C) 2001 Elsevier Science Ltd. All rights reserved.