We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa 2Se 4 and ZnGa 2S 4. Most of the A II B 2 III C 4 VI compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group I 4 ¯ where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry I 4 ¯ 2 m (defect stannite), has been reported. For ZnGa 2Se 4 we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group I 4 ¯ for the investigated crystals of ZnGa 2Se 4. Regarding ZnGa 2S 4 we have found three modes exclusively showing Raman activity (2 A⊕1 B 1), and only eight modes showing infrared as well as Raman activity (3 B 2⊕5 E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of A II B 2 III C 4 VI compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa 2S 4 most likely crystallizes in space group I 4 ¯ 2 m .