Interference spectra of layered ZnIn₂S₄ crystals were investigated for samples of different thicknesses (7.5–900 μm). Spectral dependences of refractive indices (n^a and n^b) for light waves with different polarizations were calculated and their intersection observed at energy E₀ ∼ 2.38 eV. The refractive indices difference (Δn = n^a − n^b) was determined at energy range 0.8–3.0 eV. Features observed in reflection and absorption spectra were attributed to the band-to-band electron transitions at k = 0. Magnitudes of valence bands (V₁, V₂ and V₃, V₄) splitting due to crystal field and spin–orbital interaction were determined. Electron transitions in energy interval 2–6 eV were identified on the basis of available theoretical band structure calculations.