Molecular modeling and diffraction simulation studies of the biphenyl-based poly(ether ketone)s [O−Ar−Ar−O−Ar−CO−Ar−Ar−CO−Ar−]n (3) and [−O−Ar−Ar−CO−Ar−]n (4, Ar = 1,4-phenylene), coupled with data obtained from an oligomer single-crystal study, have enabled their crystal and molecular structures to be determined from X-ray powder data. A crystallographically disordered structure in which the ether and ketone linkages are distributed randomly over the bridging positions of the polymer chain is potentially available to both polymers and undoubtedly occurs in their random 1:2 copolymer (5). The disordered structure is monoclinic (rather than orthorhombic as has previously been proposed), in space group P21/b, with two chains per unit cell. Rietveld refinement of this structure for polymers 3 and 4, based on X-ray powder data, gave final agreement factors (Rwp) of 5.7% and 6.6%, respectively. Analysis of X-ray fiber data by diffraction simulation methods suggests that the disordered structure always predomin...