A detailed analysis of the packing of polymer chains of p-hydroxybenzoic acid in the low- and high-temperature range is presented. Chain conformations and the orientation of the chains in the unit cell have been determined by a molecular mechanics approach, based on a two-step procedure consisting of grid-scan calculations and a Monte-Carlo-type study. Intramolecular and intermolecular degrees of freedom were considered simultaneously in the optimization process. The low-temperature structures of polymer samples (phases I and II) were found to be more stable than the structures observed in oligomer samples. In phase I, the cell parameters that are perpendicular to the chain axis tend to be smaller in the calculations than in experiments. Furthermore, the calculations suggest the existence of polymorphism at a microscopic level due to the coexistence of several packing possibilities with comparable stabilities.