We have measured far-infrared and infrared reflectivity as well as Raman scattering in an [(InP)5(In0.49Ga0.51As)8]30 superlattice grown by molecular beam epitaxy. A numerical model for calculating the reflectivity coefficient for complex systems which includes superlattice, buffer layer, and substrate has been developed. The far-infrared reflectivity spectra consists of the superlattice confined and interface modes as well as the modes from the buffer layer (In0.49Ga0.51As) and the substrate (InP). In the infrared spectral range above 1000 cm−1 we observe only interference fringes from the buffer layer. A good agreement between calculated and experimental spectra is achieved. The folded longitudinal acoustic phonon doublet appears at about 39 cm−1 in the Raman scattering spectra. The frequency agrees well with a continuum model calculation. In the optical phonon spectral region we observe confined modes corresponding to both constituents. The modes representing vibrations of atoms at both interfaces: InP/InGaAs (230 cm−1 mode) and InGaAs/InP (240 and 260 cm−1 modes) have also been observed. The geometrical parameters of the sample, obtained from the fitting of the reflectivity data, agree well to the values of the layer thickness obtained by double crystal x-ray diffraction.