The polarised i.r. reflectivity spectra of single crystals of the non-linear chalcopyrite materials ZnGeP2 and CdGeP2 have been measured over the spectral range 40-700 cm-1. Analysis of the spectra using classical dispersion theory and the Kramers-Krönig technique has yielded, in each material, five modes with frequencies between 80 and 385 cm-1 with the polarisation perpendicular to the optic axis. Parallel to the optic axis, two modes have been observed with frequencies between 300 and 400 cm-1. The phonon dispersion curves of the isoelectronic analogue GaP have been used, together with a group theoretical analysis, to assign specific atomic displacements to the observed modes. © 1974.