We are studying the microstructure of PPTA-co-XTA copolymer fibers. XTA is a variant of terephthalic acid (TA) with a benzocyclobutene (BCB) cross-linking moiety which becomes reactive above the synthesis and processing temperatures but below the degradation temperature. The fibers were dry-jet wet spun from lyotropic liquid crystalline solutions and heat-treated at various temperatures to induce structural reorganization and cross-linking. The microstructure was examined bywide-angle X-ray diffraction and molecular modeling. The copolymers retain the ability to crystallize and form well-oriented fibers. The BCB units are accommodated by a gradual increase in the distance between hydrogen-bonded sheets, the a dimension of the two-chain unit cell. The BCB units of high XTA content copolymers segregate into (100) planes. The a, b, and c dimensions of the unit cell of un-cross-linked PPXTA are respectively 0.91, 0.47, and 1.24 nm. Cross-linking does not significantly change the diffraction patterns, suggesting that the reaction may occur preferentially within the grain boundaries between crystallites.