Self-assembled monolayers of decanethiol on Au(111) have been investigated by means of low energy He beam scattering. In particular the specular reflectivity has been measured and features associated with resonances due to bound states of He atom interacting with the CH3 groups of the standing-up organic molecules have been observed. The analysis of the data has allowed the determination of four bound state levels and the He-surface interaction potential averaged over the unit cell has been estimated. (C) 2003 American Institute of Physics.