Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl4In3GaS8-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52eV, respectively. The rate of change of the indirect band gap with temperature dE(gi)/dT = -6.0 x 10(-4) eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E-gi(0) = 2.44 eV. 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 4.87 eV, 26.77 eV, 8.48 x 10(13) m(-2) and 2.55, respectively. (C) 2008 Elsevier Ltd. Ail rights reserved.