The polarized IR reflectivity of the ternary chalcopyrite semiconductor CdGeAs2 has been measured in the spectral region 50-500 cm-1 at room temperature. Five of the six Λ5 modes and two of the three Λ4 modes have been observed. The data have been analyzed ussing classical dispersion. The phonons are compared with those of GaAs and InAs. We have applied a Keating model to complete the experimental results published on both compounds. After comparison, it has been found that some modes of CdGeAs2 are close to GaAs, others are close to InAs and a third group are intermediate between both compounds. © 1990.