Vibrational Raman scattering and x-ray diffraction have been performed on GaSb/InAs superlattices. The composition of the ${\mathrm{As}}_{\mathit{x}}$${\mathrm{Sb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ plane that connects the InAs and GaSb layers has been varied by controlled growth techniques. When x is equal to 0 or 1, the InAs and GaSb layers are connected with InSb or GaAs bonds, respectively. Planar vibrational modes (PVM's) localized at the interface are observed. The energies of the GaAs-like and InSb-like PVM's are studied as a function of x and are compared with results of a random-element isodisplacement model.