Homo- and copolyesters of derivatives of hydroxyazobenzenecarboxylic acid with various percentage compositions of m-hydroxy benzoic acid (m-HBA)/p-hydroxy benzoic acid (p-HBA) were synthesized and characterized. The properties of the copolyesters were compared with their corresponding homopolyesters. The solubility of the copolyesters with m-HBA increased because of the decrease in the rigidity of the polymer chain attributed to the introduction of nonlinear molecules, whereas the solubilities of the copolyesters with p-HBA changed only slightly compared to their corresponding homopolyesters. Thermal and phase behaviors of the polymers were characterized by TGA, DSC, and polarizing light microscopy (PLM) methods. Above 30% composition of m-HBA, the thermal stability of the copolyesters with m-HBA decreased compared to that of the homopolyester P1, whereas the copolyesters of p-HBA possessed greater thermal stability than that of their homopolyesters at all compositions. The introduction of the long, flexible alkyl side chain laterally to the backbone of the azobenzene moiety drastically reduced the transition temperature of the homopolyester, but without destroying the mesophase. The effect of copolymerization on liquid-crystalline behavior and transition temperature of the copolymers was discussed. (C) 2003 Wiley Periodicals, Inc.