The mechanical properties of aligned tapes and monofilaments of a random copolyester, containing 17.5% of biphenol (COTBP), were investigated using macroscopic and X-ray techniques on both unannealed and annealed samples. The results were compared with similar studies of a random copolyester of hydroxybenzoic acid and hydroxynaphthoic acid (BNA/HNA). Whereas in the HBA/HNA copolymer on the scale observed by X-ray diffraction, the chain is sinuous, with an increasing sinuosity upon annealing, the COTBP is straight and no effect is seen upon annealing. The macroscopic moduli of both polymers increases with annealing. This is attributed to an increase in the shear modulus caused by an increase in the packing density of the chains. (C) 1997 Elsevier Science Ltd.