Structural and optical properties of 75 mol % Ga2Se3 - 25 mol % Ga2S3 system of single crystals were investigated by experimental techniques of x-ray diffraction (XRD), energy dispersive spectroscopy, Raman spectroscopy and ellipsometry. XRD pattern indicated that the studied compound has crystalline nature with cubic structure. Vibrational modes in the crystal were revealed using Raman spectroscopy experiments in the 90-450 cm(-1) frequency range and nine modes were observed in the spectrum. Ellipsometry measurements were utilized in the 1.2-6.2 eV range to get spectral dependencies of optical constants; complex dielectric function, refractive index and extinction coefficient. Under the light of fundamental expressions and models, refractive index and extinction coefficient spectra were analyzed to get various optical parameters of the single crystal.