High resolution helium atom scattering (HAS) has been used to measure the adsorption, diffraction and low frequency vibrations of acetic and propionic acids adsorbed on the Pt(111) surface at 170 K. Acetic and propionic acid adsorb at 170 K as isolated molecules, initially, with evidence for the dissociation of a fraction of the molecules to produce CO. At the completion of the monolayer helium diffraction shows that the molecules are locally ordered with characteristic length scales of 3.6 Angstrom and 4.5 Angstrom. By comparing the HAS time-of-flight spectra for the hydrogenated and deuterated adsorbed molecules the parallel wagging mode frequencies for the acetate and propionate bidentate components could be determined to be 12.6 meV and 8.9 meV, respectively.