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The structure of the composite crystal urea/heptadecane was determined by single-crystal X-ray diffraction. The lattice constants of the hexagonal substructures are a = 8.218 (2), b = 8.218 (3), ch = 11.017 (3) Å for the urea (host) structure and cg = 23.713 (1) Å for the heptadecane (guest) structure, respectively. The (3+ 1)-dimensional space group is P6122(00\rho)011. Refinements on 786 reflections converged to wR = 0.0232 for 608 main host reflections, wR = 0.1506 for 10 main guest reflections, wR = 0.0328 for 49 common main reflections and wR = 0.753 for 119 pure satellite reflections. A comparison of the refinement of only the urea substructure and a refinement of the whole composite crystal, including the guest subsystem and the mutual modulations of both subsystems, shows that the main reflections, previously assigned to the urea host exclusively, are affected by a nonnegligible contribution of the heptadecane satellite scattering. The modulation of the guest structure has a maximum when the CH2 groups of the heptadecane molecule are facing the channel walls at heights corresponding to troughs in this wall. Thus, the modulation of the guest structure can be interpreted by an adaptation of the guest molecules to the host structure. The modulation of the host structure was found to be very weak, as the satellite scattering of the host structure is low. \psi-scans of the 00 l reflections revealed that the observed violation of the 61 screw axis extinction rule of the host structure and guest structure modulation can be explained by considerable umweganregung.

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