The optical properties of Tl2In2S3Se layered single crystals have been studied by means of transmission and reflection measurements in the wavelength range of 450-1100 nm. The analysis of the absorption data revealed the presence of both optical indirect and direct transitions with energy band gaps of 2.16 and 2.42 eV, respectively. The observed coexistence of indirect and direct energy band gaps is related to the anisotropy of the TI-containing layered crystals. The dispersion of the refractive index is discussed in terms of the Wemple-Di Domenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.78 eV, 43.58 eV, 13.18 x 10(13) m(-2) and 3.18, respectively. From X-ray powder diffraction study, the parameters of monoclinic unit cell were determined. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.