Silver gallium (indium) ternary selenides (tellurides) single crystals were studied through infrared reflection in the frequency range of 80-500 cm(-1). These spectra presented four infrared-active modes for studied crystals. Spectral dependencies of optical constants were computed from reflectivity spectra. The frequencies of transverse and longitudinal optical modes, damping constants and oscillator strengths were also evaluated. By replacing light selenium anions by heavier tellurium ones in Ag3Ga5Se9 crystal and by substitution of light gallium cations by heavier indium ones in Ag3Ga5Te9 crystal all the bands shift towards low frequencies. The bands observed in IR spectra of studied crystals were assigned to various vibration types (valence and valence-deformation). Crystal structure and atomic composition ratio of the constituent elements in Ag3Ga5Se9, Ag3Ga5Te9 and Ag3In5Te9 crystals were revealed by structural characterization techniques of X-ray diffraction and energy dispersive spectroscopy.