The structure of a side-chain liquid crystalline polysiloxane containing two oligo(oxyethylene) units and (R)-1-methylheptyl 4-(4-hydroxybiphenyl-4'-carbonyloxy) benzoate mesogens (PS221B) has been studied using X-ray diffraction patterns and molecular modeling. Ten periodic simulated cells with equilibrated structures are generated, each containing 35 mesogenic groups and a polysiloxane backbone. The simulated X-ray diffraction patterns are in agreement with those obtained experimentally. The calculated width of the sublayers of the polysiloxane backbone is about 5.63 angstrom along the z-axis. The distributions of the dihedral angles of the polysiloxane backbone in the equilibrated structures indicate that the backbone has a higher probability distribution in the trans state. The distributions of the dihedral angles at the bonds in the aromatic core of the mesogenic groups reveal high probabilities of the trans and cis placements. The local order due to the interactions between the aromatic cores is in the range of 3-5 angstrom. The order of the radial distribution function vanishes beyond this range. (c) 2007 Elsevier Ltd. All rights reserved.