The inelastic neutron scattering spectra of a set of homologous lamellar molecular crystals containing two-dimensional hydrogen bonded sheets of guanidinium ions and sulfonate moieties of organomonosulfonates have been measured. The spectra were collected in the 35-105 meV range using a filter analyzer spectrometer and in the 0-35 meV energy range using a time-of-flight instrument. Parallel and perpendicular vibrational modes, associated with hydrogen displacements with respect to the hydrogen-bonded sheets, were analyzed between 35 and 105 meV by employing different scattering geometries. Comparison of the filter analyzer spectra of the isomorphous guanidinium salts of methanesulfonate, triflate and dithionate enabled assignment of specific modes while providing direct unambiguous measurement of the effect of the organic substituent on the strength of the (guanidinium)N-H .O-S(sulfonate) hydrogen bonds. Comparison of the time-of-flight spectra of the three compounds revealed that a mode observed at 7 meV corresponded to rotational motion of the methyl group. Density functional theory reproduced the experimental spectrum of guanidinium methanesulfonate, enabling assignment of the lattice vibrational modes. The scattering data compare well with a previously reported vibrational mode analysis of structurally homologous guanidinium nitrate.