Raleigh and Mandelshtam-Brillouin light scattering has been investigated in chalcogenide glasses of the (As₂S₃)_xI_1−x system for x = 1.00−0.80. Longitudinal hypersound velocities, adiabatic elastooptical constants (p₁₂)_ad at frequencies of ∼15 GHz, and Landau-Placzek ratios R_LP are found and scattering losses are calculated. Significant iodine additives are shown to result in the appearance of different features in the behavior of concentration dependences of the investigated parameters.