The collective vibrational excitations of two different crystalline monolayer phases of ethane (C2H6) adsorbed on the graphite (0001) surface have been investigated theoretically and experimentally. The monolayer phases studied are the commensurate 7/8 ×4 structure in which the ethane molecules lie on their side in a herringbone arrangement and the commensurate 7/8 × 7/8 structure in which the molecules stand on-end with the C–C bond perpendicular to the surface. Semiempirical atom–atom potentials have been used to model the intermolecular and molecule–substrate interactions in calculations of the monolayer structure and lattice dynamics. The observed inelastic neutron scattering spectra below 4.1 THz are compared with calculated one-phonon incoherent cross sections. The calculated cross sections qualitatively reproduce the rather different spectra of the two phases and enable an identification of the observed vibrational modes. Moreover, they suggest that an important feature of the herringbone phase dynamics is a coupling of the lowest-frequency librational mode to the vibratory mode perpendicular to the surface. Calculations of the phonon dispersion relations, the phonon density of states, and the phonon contribution to heat capacities of the two monolayer phases are also presented and discussed. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.