Phonons vibrational frequencies for Hg1-xCdxSeHg1-xCdxSe (x - 0.5) crystalline alloys were theoretical and experimentally studied. Two simple expressions that summarize the optical modes behavior with composition were derived. Such that the behavior of the vibrational properties of II - VI and III - V semiconductors at the ? point can be expressed as function of the product of the ionic charges (Z1Z2) and the nearest neighbor distance, d. A good agreement has been found between the observed and calculated values of the vibrational frequencies Vlo for the ternary Hg1-xCdxSeHg1-xCdxSe alloys. Furthermore, our calculations suggest a non typical behavior of the acoustic vibrational modes similar to the one reported for HgSe. Besides, we also report the theoretical dependence of the phonon dispersion relation as function of composition.