Raman and infrared (IR) reflection characteristics were investigated in the frequency region of 100-450 cm(-1) for (Ga2S3)(x) - (Ga2Se3)(1-x) mixed crystals for compositions of x increasing from 0.0 to 1.0 by intervals of 0.25 obtained by Bridgman crystal growth technique. In the Raman spectra of these crystals four dominant peak features were observed while two bands were detected in the IR spectra of interest samples. Kramers-Kronig dispersion relations applied to IR spectra presented the frequencies of transverse optical modes. The compositional dependencies of revealed Raman- and IR-active mode frequencies on (Ga2S3)(x) - (Ga2Se3)(1-x) crystals were established. One-mode behavior was displayed from indicated dependencies.