Optical properties of the TlGaxIn1-xS2 mixed crystals (0 <= x <= 1) have been studied by means of transmission and reflection measurements in the wavelength range of 400-1100 nm. The optical indirect band gap energies were determined through the analysis of the absorption data. It was found that the energy band gaps increase with increasing of gallium atoms content in the TlGaxIn1-xS2 mixed crystals. From the transmission measurements carried out in the temperature range of 10-300K, the rates of change of the indirect band gaps with temperature were determined for the different compositions of mixed crystals studied. (c) 2010 Elsevier B.V. All rights reserved.