Several series of polyester imides (PEIs) were prepared by melt polycondensation of trimellitimidobenzoic acid and acetylated, substituted hydroquinones as well as by solution polycondensation of corresponding dichlorides with hydroquinones. The influence of either phenyl- or tert-butyl substituents at the hydroquinone unit on the hydrodynamic (diffusion and viscometry) and optical [flow (FB) and electrical (EB) birefringence] properties of solutions of these PEIs were investigated. Independent methods for quantitative evaluation of the length of the statistical Kuhn segment A gave values of 14.5 nm (translational friction and electrical birefringence) and 12.0 nm (flow birefringence), respectively. The equilibrium rigidity of PEls with phenyl and tert-butyl lateral substituents at the para-aromatic chain fragment was comparable. Conformational properties of the PEls were analyzed, and the conjugation energies of the ester and amide groups for para-aromatic polyesters, PEls, and structurally similar polyamidobenzoxazoles and polyamidobenzimidazoles were compared. The introduction of the phthalimide ring leads to a decrease of conjugation energy and deviation of the ester group from coplanarity. The Kuhn segment values obtained were useful for calculations of phase diagrams of semiflexible segmented block copolymers. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 12-24, 2004.