This study shows that the structure of the poly(diphenylsiloxane) (PDPhS) backbone is quasi-planar rather than helical. The nearest-neighbor chains in the crystal are packed in a hexagonal configuration. Using molecular mechanics calculations and analysis of X-ray powder diffraction data, it has been determined that the crystalline structure of PDPhS has Pbn2(1) symmetry and the backbone consists of siloxane bonds in a quasi cis-trans sequential conformation. The deviation of each bond from the planar configuration is about 15 degrees. The backbones of the two chains in the unit cell are antisymmetrical. The Si-O-Si bond angle is 145 degrees and the torsional angles of the vicinal phenyl groups relative to each Ether are 125 and 72 degrees. The orientation of the phenyl groups in the crystal consists of parallel and perpendicular nearest neighbors. The calculated X-ray results are able to explain the main features of the experimental pattern in terms of position and intensity of the peaks. Molecular mechanics calculations suggest that the density of ideal crystals of poly(diphenylsiloxane) at low temperatures should be in the range 1.26-1.3 g/cm(3), whereas the reported experimental value is 1.22 g/cm(3). A new set of molecular mechanics parameters for the UFF method to more accurately describe polysiloxanes in the condensed state is given.