We report a Raman study of the lattice vibrations of the quaternary compound In1−xGaxAsyP1−yover the whole range of compositions lattice‐matched to InP. The major bands in the spectra show apseudo‐two‐modebehavior involving the optical phonons of InGaAs and InP. A low‐frequency structure is attributed to disorder‐induced scattering by acoustical vibrations. Weaker bands are assigned to Ga‐P and In‐As pair vibrations.