The structures of poly(p-hydroxybenzoic acid) (PHBA) at the two high-temperature phase transitions (ca. 340 and 430 °C) have been investigated by X-ray powder diffraction methods (X-ray tube focusing and synchrotron parallel beam), chain conformation/packing models, and dielectric measurements. The structure of PHBA between ca. 340 and 430 °C, derived from the X-ray patterns and available electron diffraction results, is satisfactorily accounted for by the following model: orthorhombic cell, a = 9.24 Å, b = 5.28 Å, and c = 12.50 Å, with two successive phenylene planes along the c-axis rotated 60° from each other in the unit cell; the phenyl groups in the a-b plane are oriented in an ordered herringbone-type packing in each phenyl layer with their planes 60° and 120° with respect to the a-axis; and each chain segment (of two monomer repeats) in the unit cell is positioned randomly in the a-b projection among the two different sites involving a twofold axis of reorientation through an angle π. The dielectric constants at high frequencies (100 kHz) indicate rapid chain mobility accompanying this high-temperature phase. This structure of PHBA therefore follows quite well the model of the smectic-E phase of small rodlike molecules. The second transition around 430 °C involves no change in the packing order along the chain axis or chain conformations but only the loss of long-range phenyl orientation order in the a-b plane, very much like the smectic-E to smectic-B transition of small rodlike molecules. The coherence length along the chain axis of PHBA crystals at room temperature is estimated to be ca. 2700 Å, according to the Sherrer formula, and this packing order along the chain axis is maintained throughout the two high-temperature transitions. Relevance of the smectic-type high-temperature structures of PHBA to the molecular order in aromatic copolyesters comprising majority HBA monomers is then discussed. © 1990, American Chemical Society. All rights reserved.