The vibrational properties of undoped α- and β-GaN epilayers on GaAs substrates have been investigated at 300 and 77 K by far-infrared Fourier transform spectroscopy using polarized oblique incidence reflectivity and Raman spectroscopy. These techniques have enabled us to determine the transverse- and longitudinal-optical phonon frequencies at the center of the Brillouin zone for propagation parallel and normal to the epilayers, as well as to investigate a number of interesting interface modes. Comparison of the phonon properties of a selection of GaN epilayers deposited by molecular-beam epitaxy on GaAs and GaP substrates indicates that their properties are strongly influenced by the substrate material and orientation. The presence of Berreman interface modes in the spectra is evidence of the presence of a boundary layer at the base of the GaN epilayer adjacent to the substrate. By modeling the boundary layer as a very thin ((Formula presented) in thickness) highly disordered region at the base of the GaN layer, all features in the measured spectra are satisfactorily assigned. © 1998 The American Physical Society.