Abstract Low energy helium atom scattering has been used to characterize monolayers of HCl adsorbed onto single crystals of graphite. The unit cell of this system is described by real space lattice vectors 3.80 Å in length, separated by 120°. The monolayer's lattice is rotated 30° with respect to the substrate. The attenuation of the He beam scattered in the specular direction due to the thermal motion of the HCl molecules perpendicular to the surface, can be described as due to a collection of Einstein oscillators with a common, characteristic frequency of 7.8 × 10 12 rad s . A polar plot i.e. a plot of the specular intensity as a function of the incident angle, has been compared to the predictions of a close-coupled scattering program, that uses realistic He-surface interaction potentials which include the contributions from the atom-adlayer and atom-substrate interactions, along with the relevant three-body interaction terms. The existing HeHCl spherically averaged potentials do not give good agreement with our data although reducing the well depth by 8.5% improves the agreement slightly. Since the HeHCl interaction in the “T” geometry is less strong than that along the molecular axis (and therefore than the spherically averaged interaction) our measurements may suggest that the HCl molecules are preferentially aligned with their dipole axes parallel to the surface.