The effect of the ether group of the spacer of poly[oxybis(trimethylene) pp'-bibenzoate] (PDTMB) on the thermotropic behavior of this polyester has been analyzed by comparing the properties of PDTMB with those of poly(heptamethylene p,p'-bibenzoate) (P7MB), the analogue polybibenzoate with an all-methylene spacer. The ability to develop three-dimensional order from the mesophase is greatly diminished by the presence of the oxygen atom. Moreover, P7MB exhibits a monotropic behavior as the crystal, on heating, is transformed directly into the isotropic melt, while PDTMB appears to be a ''regular'' thermotropic polymer. The oxygen atom, however, seems to have almost no influence on the stability of the mesophase, and both polyesters present similar thermodynamic parameters for the liquid crystalline phase. Moreover, theoretical studies were carried out on the two polymers to estimate the angular correlations between two successive mesogenic groups, chain sequence extension and mean-square dipole moment distributions, and internal energies associated with the different conformers in the melt. Very similar quantities have been found for both polybibenzoates regarding these conformational characteristics that can affect the formation and stability of the respective mesophases.