We have used the hybrid density functional B3LYP method in combination with cluster surface models to study the adsorption of C2H2 on the Si(100)-2 x 1 surface. Four bonding configurations of C2H2 adsorption on a single Si dimer and on two neighboring dimers have been found. Two bonding configurations with C2H2 di-σ bonds result in high and comparable adsorption energies (> 60 kcal mol-1), whereas the two bonding configurations with C2H2 tetra-σ bonded over two neighboring Si dimers give lower, but substantial, adsorption energies centering around 44 kcal mol- 1. The calculated vibrational frequencies for the bonding configurations with C2H2 di-σ bonded structures are found to be in reasonable agreement with the experimental HREELS spectra reported in the literature (M. Nishijima, J. Yoshinobu, H. Tsuda and M. Onchi, Surf. Sci., 1987, 192, 383; C. Huang, W. Widdra, X. S. Wang and W. H. Weinberg, J. Vac. Sci. Technol. A, 1993, 11, 2250).