Theoretical infrared (IR) absorption spectra were calculated for p-tert-butylcalix[4]crown-6-ether (1) in the cone conformer and its ethyl ammonium complex. The IR spectra were obtained by restricted Hartree-Fock (RHF) calculations with the 6-31G basis set. For the purpose of an absorption band assignment on the host molecule, guest molecule, and complexes thereof, we discussed a way to distinguish a specific molecule by comparing the calculated vibrational spectra. The theoretical result for the host molecule I and its ethyl ammonium complex were preliminarily compared with the experimental result, and found that the calculated result agrees well with the features of the experimental spectra. (C) 2003 Elsevier Science B.V. All rights reserved.