The optical properties of TlGaxIn1-xS2 mixed crystals have been studied through transmission and reflection measurements in the wavelength range 400-1100 nm. These measurements allowed determination of the spectral dependence of the refractive index for all compositions of the mixed crystals studied. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The compositional dependences of the refractive index dispersion parameters (oscillator energy, dispersion energy and zero-frequency refractive index) were revealed. (C) 2010 Elsevier Ltd. All rights reserved.