The van der Waals interactions of acetylene with hydrogen-terminated diamond single crystal surfaces have been explored using infrared polarization spectroscopy at 83 K. By observing infrared spectra of the antisymmetric stretch of C2H2, the molecules are found to be adsorbed first on defect sites. This is followed by the adsorption on flat terraces that yields a single absorption at v(0) = 3225.6 cm(-1) with a fwhm = 5.4 cm(-1). The adsorbates lie nearly flat on the surface with a monolayer packing density of 5 x 10(14) molecules/cm(2) and an adsorption energy of 23 kJ/mol. Isotope mixture experiments were also performed to provide information on the structure of the adsorbed phase. From the narrow width (fwhm <3.0 cm(-1)) of the C2HD features and the result of exciton coupling calculations, it is suggested that the adsorbed layer forms a highly orientationally ordered superlattice. Two possible structures, (2x1) and (2x2) in (root 3x root 3)R30 degrees array, of the superlattice are proposed.