The exact location and orientation of dynamic species in structural studies continues to be a serious challenge, yet it is of paramount importance in modeling guest–host interactions so as to improve our understanding of the multiple weak interactions that govern many chemical and biological processes. The acetone guest in the tBC (p-tert-butylcalixarene) host presents such a challenge, as initial guest positions obtained from single crystal X-ray diffraction (XRD) are incompatible with the 2H NMR spectrum. A detailed consideration of the diffraction data showed that more complicated structural models could be constructed that were consistent with the NMR data and still yielded satisfactory diffraction residuals. These models agree that one acetone methyl group is inserted into the deep cavity, and that it exchanges with the second methyl group outside. The outside methyl group in turn can switch positions with the carbonyl group, but the distribution of the methyl and carbonyl groups over the two sites is not equal. One factor that poses additional difficulty in deciding between models is whether the actual space group is tetragonal (P4/n), or twinned monoclinic (P2/n). All of the structural models considered here disagree substantially with the one proposed in an earlier publication.