The authors report ellipsometry measurements on polycrystalline samples of NiMnSb and PtMnSb at room temperature under ultra-high vacuum conditions. From these measurements the optical constants of the two materials were determined in the energy range 0.5-5.3 eV. The measured optical conductivities are in reasonable agreement with the optical conductivities calculated from the band structures by de Groot (1983). The band gap for the minority-spin direction is clearly observed and can be brought into agreement with the band-structure calculations by consideration of spin-orbit splitting.