The reflectivity of TmS single crystals has been measured between 3 meV and 12 eV at room temperature and at 6 K, Besides 3p(6)-5d interband transitions a plasma edge due to free carriers is observed. In the middle infrared (near 50 meV) two sharp lines are found which are interpreted as transitions from the Fermi level into empty crystal field split 4f(1 3) states. A Bremsstrahlen isochromat spectroscopy (BIS) measurement supports the assumption that the empty 4f(1 3) state is close to the Fermi energy. (C) 1998 Elsevier Science B.V. All rights reserved.