AbstractIn a preceding paper a set of coupled equations describing the effect of a disorder mechanism on the dispersion of optical phonons and polaritons was given. These equations are valid for the paraelectric region of ferroelectric crystals. The set of coupled basic equations is extended to be valid also for the ferroelectric region. The properties of the different wave types as solutions of these equations are very similar to those of the paraelectric region. At the Curie point the coupling leads in general to a hard core frequency for which a closed expression is given. However, in the limiting case of small Debye relaxation times the theory results in a soft‐mode‐like behaviour for the transverse optical phonons. The algebraic and numerical results are discussed in detail.