The optical constants of clean thallium single crystals have been determined in the energy range 0.6 to 5.0 eV, at room temperature and at 80K, for polarizations parallel and perpendicular to the hexagonal axis. The results are discussed in terms of direct interband transitions with constant transition probability matrix elements. The joint density of states calculated from the band structure of Holtham and Priestley (1971) predicts the experimentally observed structure in omega ² epsilon ₂ at h(cross) omega =1.7 eV; however at higher photon energies the resemblance between the calculated and experimental structure in the optical spectra is found to be rather poor.