Microstructural information about the nature of the polymer-water interaction in rigid-rod polybenzimidazole polymer fibers (PBI) is derived from the X-ray crystallographic analysis of 1,7-dihydro-2,6-diphenylbenzo-[1,2-d;4,5-d']diimidazole tetrahydrate, C20H14N4.4H(2)O. MW = 382.20 amu, a model compound for poly(p-phenylene-benzobisimidazole) (PBDI). The model compound crystallizes in a monoclinic crystal system, space group P2(1)/c, with n = 9.008(2) Angstrom, b = 24.967(7) Angstrom, c = 9.870(5) Angstrom, beta = 119.82(3)degrees, and Z = 4. Molecules pack in a herringbone fashion, interspersed with a network of solvent water molecules. Hydrogen-bonded water molecules bridge molecules of the model compound that are related by an inversion center. Each water molecule acts both as a hydrogen bond donor and as a hydrogen bond acceptor. A plausible model for the packing of chains in heat-treated fibers elf PBDI would involve polymer chains extending in the direction of elongation, in the manner found for polybenzothiazoles (PBZT) and polybenzoxazoles (PBO), but with a network of hydrogen-bonded water molecules providing strong lateral interactions between polymer molecules. A comparison is made with the structure of poly-{2,6-diimidazo[4,5-b:4',5'-e]pyridinylene-1,4(2,5-dihydroxy) phenylene} (PIPD or 'M5' fiber) since the enhanced compressive behavior of PIPD is attributed to a hydrogen-bonding network between polymer chains.