The structural properties of the hexadecane/urea inclusion compound have been determined, using Rietveld refinement, from synchrotron X-ray powder diffraction data recorded al temperatures above and below the phase transition temperature (ca. 150 K) for this inclusion compound. Structural characterization of the low-temperature phase by single-crystal diffraction techniques is limited by the fact that single crystals of the inclusion compound become multiply twinned on cooling below the phase transition temperature. The structural properties determined for the high-temperature phase at ambient temperature are in agreement with those reported previously from single-crystal X-ray diffraction data; the urea molecules form a hexagonal host tunnel structure, with an incommensurate relationship between The periodicities of the host and guest substructures along the tunnel axis, The host structures in the low-temperature and high-temperature phases are sufficiently similar that the high-temperature host structure can be used as the initial structural model for Rietveld refinement of the low-temperature phase. The urea tunnel structure in the low-temperature phase (studied at 120 K) is orthorhombic (P2(1)2(1)2(1); a = 10.98 Angstrom, b = 13.89 Angstrom, c = 8.25 Angstrom) and represents distorted form of the hexagonal structure of the high-temperature phase. This represents the first accurate and reliable report of the low-temperature structure of an alkane/urea inclusion compound, The observed distortion of the urea tunnel is consistent with the fact that the reorientational motion of the guest molecules about the tunnel axis diminishes substantially upon entering the low-temperature phase, The strategy developed in this paper for structure determination of the low-temperature phase of the hexadecane/urea inclusion compound will have wider application other incommensurate solid inclusion compounds that exhibit phase transitions at low temperatures. This approach is particularly important for cases in which single crystals of the inclusion compound become multiply twinned upon entering the low-temperature phase.