Infrared reflection spectra are registered in the frequency range of 50-2000 cm(-1) for Ag3Ga5S9 and Ag3In5S9 single crystals grown by Bridgman method. Four infrared-active modes are detected in spectra. By replacing the gallium atoms by indium ones in Ag3Ga5S9 crystal, the observed bands shift to low frequencies. Spectral dependence of optical parameters; real and imaginary parts of the dielectric function, the function of energy losses, refractive index, absorption index and absorption coefficient were calculated from reflectivity experiments. The frequencies of transverse and longitudinal optical modes and oscillator strength were also determined. The highest frequency bands observed in an infrared spectra of studied crystals were tentatively assigned to the antiphase vibration of the trivalent cation and anion sublattices. (C) 2016 Elsevier GmbH. All rights reserved.