Rigid rod copolyesters derived from 2,5-bis(pentyloxy)terephthaloyl dichloride (3), 4,4’-dihy-droxybiphenyl (4) and trans-1,4-dihydroxycyclohexane (5) were synthesized and their thermal behavior analysed. The structures were characterized by H-1-NMR, GPC and elemental analyses; their thermal behavior was studied by DSC measurements, microscopy under polarized light and thermogravimetric analyses. The composition of the copolyesters with 30, 50 and 70 mol % 5 was not as expected, due to the differing reactivities of the diols 4 and 5 during the polycondensation. This led to the existence of two species of polyesters from one reaction mixture. The higher the content of 5 in the polymer, the lower the molecular weight. By increasing the amount of 5, the thermal stabilities and melting points decrease drastically, whereas the solubility increases. The copolyesters a-g show only a low ability to form fibres which are brittle and break very easily.