A concept is presented which gives qualitative information concerning the phonon behaviour of ionic mixed crystalsAB1−xCx at certain symmetry points on the zone boundary and consequently through the zone to the center. The model assumes that the end-member phonons and the impurity-modes ofAB:C andAC:B are known and uses the assumption that mixed crystal phonons behave symmetry-wise as the corresponding end-member phonons. The concept is demonstrated for several cases of mixed crystals.