The interaction of ethyne with vacuum-dehydrated CuNaY zeolite has been investigated by means of Fourier transform infrared spectroscopy and cluster model density functional theory calculations. The spectra show three bands: two which are symmetry-forbidden for the free molecule but become allowed in the adsorbed state (the nu(1) CC stretching mode at similar to1832 and 1814-1812 cm(-1), and the symmetric nu(2) CH stretching mode at 3271 and 3251 cm(-1)), and the allowed antisymmetric nu(3) CH stretching mode at 3202 and 3174 cm(-1). They all appear in pairs. The two components of the bands, which are all strongly shifted to lower wavenumbers relative to the free molecule, are attributed to two inequivalent Cu+ adsorption sites. On increasing C2H2 pressure, the more strongly shifted components appear first and acquire significantly higher intensity, indicating that they must correspond to the more strongly bound site. Analysis of the geometry of the complexes representing ethyne adsorbed on Cu at both sites yields a four-fold coordination of copper, including two oxygen atoms and side-on bound C2H2, with CC bonds elongated by 0.03 Angstrom and CCH angles reduced by at least 14.7degrees. The calculated binding energies of the two ethyne-copper complexes are significantly different due to the energy necessary to reach the geometry within Cu(SII)-C2H2 from the relaxed Cu(SII) cluster. C2H2 is more strongly adsorbed on Cu(SIII) (in the saddle between two four-membered rings near the wall of a supercage) than on site II (above a six-ring window of a sodalite cage). Population analysis provides a net charge transfer of about 0.14e toward the ethyne. An energy decomposition method demonstrates the paramount importance of electrostatic and charge-transfer contributions to the Cu-zeolite-ethyne interaction energy. The harmonic frequency calculation suggests that the more strongly shifted band components of all stretching vibrations have to be assigned to C2H2 adsorbed on Cu+ at SIII, which is the more stable site. Apart from the overestimated Deltanu(1), values, the vibrational frequency shifts calculated for both adsorption complexes are in excellent agreement with the experimental ones.