The crystal structure and morphology in oriented solutions of the rigid polymer poly(p-phenylene benzobisoxazole) (PBZO) in polyphosphoric acid (PPA), at different degrees of exposure to moisture, were studied by transmission electron microscopy and X-ray diffraction. Two crystal solvate forms, crystalline complexes of PBZO and PPA, were found. The form I crystal solvates exhibit several sharp equatorial reflections, whereas only a few broad reflections are observed in the form II state. All crystal solvate phases studied exhibit three-dimensional crystallinity evident in sharp off-meridional reflections. This is considered to be due to unique associations between the protonated PBZO polycations and the PPA anions. Endothermic transitions observed upon heating the crystal solvates are interpreted as dissolution of the form I crystals at about 70 degrees C and crystal-crystal transformation of the form II crystal solvate to PBZO crystals, due to deprotonation above 300 degrees C. A microfibrillar morphology is observed in the oriented PBZO fibre coagulated by immersion in water. Similar microfibrils, less than 10 nm in width, are already observed in the form II crystal solvate state, obtained from the oriented solution by absorption of moisture.