Reflectance difference (RD) spectra for the a–b plane of the single crystals of Gd₅Si₂Ge₂and b–c planes of Gd₅Si₂Ge₂ and Tb₅Si_2.2Ge_1.8 were obtained in the photon energy range of 1.5–5.5 eV. Several peaks were observed for these crystals in the measured spectrum range. Similar features were observed in the RD spectra for the b–c planes ofGd₅Si₂Ge₂ and Tb₅Si_2.2Ge_1.8, while different features were observed for the a–b plane and b–c plane of Gd₅Si₂Ge₂. The RD spectra for the crystals arise not only from the surface anisotropy but also from the bulk anisotropy due to the monoclinic structure of the bulk crystal.