High-resolution He atom scattering experiments (HAS) are reported, which were carried out on a cleaved Rbl (001) surface (T s = 120K). The nearly monoenergetic ( Δv v ≈ 1%) He beam, produced from a high pressure nozzle source, was chopped into pulses for the analysis of the inelastic scattering by a time-of-flight (TOF) technique. The incident He atom energies employed in these studies ranged from 20 to 24 meV, corresponding to wavevectors of 6.2 to 6.8 Å −1. Data from single phonon annihilation and creation events were obtained in both the ▪ and ▪ surface high symmetry directions and used to construct the surface phonon dispersion curves. The experimental results are compared with previous theoretical calculations for this surface. In general, the observed Rayleigh wave is found to be in very good agreement. However, a crossing mode is found which was not predicted initially, but which has now been seen in the (001) surfaces of several other alkali halides. Perhaps most interesting is that the data support the existence of the predicted high-lying surface optical branch, S 2, pushed above the optical bulk bands because of the surface relaxation. In addition, another newly discovered branch, extending about a third of the way from the M̄ point to the \\ ̄ gG point near the top of the acoustic band is probably also due to the surface relaxation. It is possibly associated with second layer vibrations.