Reflectivity spectra of CuAlS₂, CuAlSe₂, and CuGaSe₂ crystals have been investigated in the wave number range 50 to 600 cm⁻¹ for the polarizations E ‖ c and E ⊥ c. The fundamental phonon parameters, the limiting dielectric constants ε_∞ and ε_S and the reflectivity spectra contours have been calculated by using classical dispersion relations for both E ‖ c and E ‖ c configurations. The Szigeti effective charges and the relative ion charges of Cu, Al, Ga, Se, S anions and cations have been calculated in dependence on the incident light polarization.