Investigations of the structures of the complexes formed by di-protonated oligomer molecules of poly (p-phenylene benzobisoxasole) (PBO) with charged and neutral molecules of orthophosphoric acid (OPA) by quantum-chemical and molecular mechanics methods are described. The complexes with partially compensated, fully compensated, and overcompensated PBO molecular charges as models of the polymer acid complexes in solution and in crystal-solvate phases (Phase I and Phase II) are considered. It is shown that the structures with one anion associated with two heterocycles are most energetically favorable for the complexes with a partially compensated polycation charge. The polymer molecule bending resulting from this association can contribute to an enhanced PBO chain flexibility in solution. The PBO chains in Phase I are protonated due to the presence of neutral acid molecules. The protonated state of the PBO molecules in Phase II can be explained by the presence of excess anions. (C) 2011 Elsevier Ltd. All rights reserved.