Abstract: The electronic structure and optical properties of intermetallic compounds GdTi0.05MnxFe0.95 –xSi (x = 0, 0.4, 0.6, 0.95) have been investigated in this work. Spin-polarized densities of electronic states and optical conductivity spectra have been calculated by the DFT + U method, taking strong electron correlations in the 4f shell of Gd into account. The optical properties of these materials in the energy range 0.078–4.6 eV have been measured by the ellipsometric method. The nature of the quantized light absorption has been discussed, and a comparison of the experimental and theoretical spectra of the interband optical conductivity has been used. It has been shown that the change in the optical properties of the compounds when iron is substituted for manganese can be interpreted qualitatively based on the calculations of the densities of electronic states.