CuIn5Se8 bulk single crystals were characterized using ellipsometry, photoluminescence (PL), and optical second harmonic generation (SHG). The refractive n(E) and absorption k(E) indices as functions of photon energy E are determined. The structure of the PL spectrum suggests the presence of the Cu-rich nonstoichiometric phases at the natural surfaces of the CuIn5Se8 sample. The best fit of the experimental SHG data is achieved by assuming that CuIn5Se8 belongs to the 42m symmetry group. This result is in contradiction with x-ray analysis (62m group), which testifies to the difference in surface and bulk structure.