The molecular variables which control the structure–property relationships in thermotropic liquid-crystalline polyesters are under investigation in this laboratory. A wide variety of polymers based on rigid, linear aromatic ester mesogenic units, with and without flexible or rigid non-mesogenic spacers, have been prepared and characterized for their ability to form a liquid-crystalline melt, the type of phase formed, their transition temperatures and the morphology of the mesophase. Flexible spacers reduce both the melting and clearing temperatures, and the type and length of the spacer can determine whether a nematic, cholesteric or smectic phase is formed. Variations in the structure of the rigid mesogenic group, both in the specific type and arrangement of the aromatic ester groups and the presence of pendant substituent, also cause profound changes in the properties of the mesophase melt formed. Copolymers containing rigid non-mesogenic units in random sequence distributions with closely related mesogenic units have been characterized for the effects of composition on thermotropic properties, including the parameter of ‘degree of liquid crystallinity’. Of interest in all series of polymers studied is the critical limit of non-mesogenic unit content beyond which either liquid crystallinity no longer occurs or monotropic behaviour is observed.