Monolayer films of carbonyl sulfide (OCS) on well-defined NaCl(001) single-crystal surfaces have been studied using Fouriertransform infrared spectroscopy (FTIRS) with polarized light, helium atom scattering (HAS) and low-energy electron diffraction (LEED). The monolayer growth at Tsample = 77-80 K and p(OCS) = 1 × 10 -8mbar proceeds via large islands which have the structure of the completed monolayer. For the latter, both HAS and LEED indicate a (2 × 1) superstructure, most probably with two molecules in each adsorbate unit cell. In the infrared spectrum of the OCS asymmetric stretching vibration v3, three intense and three weak absorptions are observed (FWHM < 1 cm -1). Due to the non-degeneracy of the v3 mode this observation would suggest at least three molecules in each unit cell, and challenges the conclusions drawn from HAS, LEED and the integrated infrared absorption. HAS time-of-flight spectroscopy was used to investigate the adsorbate phonon dispersion curves. Eight modes with energies below 80 cm -1 were observed along the <100> direction. The observability of some phonon modes in certain parts of reciprocal space suggests a close relation between phonon intensity and phonon symmetry.