The importance of crystallinity in the deformation processes of main-chain liquid-crystalline copolyesters is examined. Two materials are studied: one a copolymer of hydroxybenzoic acid (HBA) with 2-hydroxy-6-naphthoic acid (HNA); the other a copolymer of HBA with isophthalic acid (IA) and hydroquinone (HQ). In both cases it is observed that the transverse compressive yield stress and modulus of the polymer are optimised by annealing at about 20 degrees C below the melting temperature. In the case of the copolymer containing HBA and HNA, it is found that deformation of unannealed samples results in a pronounced loss of crystallinity. However, the effect is less marked in annealed samples, and the crystallinity is found to recover after modest annealing close to the glass transition temperature. In both polymers, evidence is found for a stress-induced solid-state phase transition, which may be similar to the martensitic phase transition proposed for polyethylene.