We have investigated the formation of self-assembled monolayers (SAMs) of 11-hydroxyundecyl decyl disulfide (CH3-(CH2)(9)-S-S-(CH2)(11)-OH, HDD) and 11-hydroxyundecyl octadecyl disulfide (CH3-(CH2)(17)-S-S-(CH2)(11) -OH, HOD) produced by supersonic molecular beam deposition (SMBD). The study has been carried out by means of helium diffraction at very low film coverage. In this regime helium single molecule cross sections have been estimated in a temperature range between 100K and 450K. The results show a different behavior above 300K that has been interpreted as the starting of mobility with the formation of two thiolate moieties either linked by a gold adatom or distant enough to prevent cross section overlapping. Finally, helium diffraction patterns measured at 80 K for the SAMs grown at 200 K are discussed and the results support the proposed hypothesis of molecular dissociation based on the cross section data. (C) 2016 Elsevier B.V. All rights reserved.