A Keating model calculation of phonon dispersion in CdGeAs2 with two bond stretching force constants between nearest neighbours and two bond bending force constants between second nearest neighbours is presented. The four parameters have been determined from a least mean square fit to our Raman data. In this study one emphasizes the relationship which connects zone centre modes in CdGeAs2 and phonons at Γ, X and W points of the Brillouin zone in the associated zincblende materials: GaAs and InAs, on the one hand, and the binary analog Ga0.5In0.5As, on the other hand. One shows that the optical modes in CdGeAs2 can be classified as GaAs-like, InAs-like and mixed GaAs/InAs-like. © 1990 IOP Publishing Ltd.