Compositional variation of the band gap energy and refractive index of TlMeX 2-type (Me ¼ Ga or In and X ¼ S or Se) layered mixed crystals have been studied by the transmission and reflection measurements in the wavelength range of 400-1100 nm. The analysis of absorption data of TlGa 1-x In x Se 2 , TlGa(S 1Àx Se x) 2 , TlGa 1Àx In x S 2 , and TlIn(Se 1Àx S x) 2 mixed crystals revealed the presence of both optical indirect and direct transitions. It was found that the energy band gaps of mixed crystals decrease at the replacing of gallium atoms by indium and of sulfur atoms by selenium ones. Through the similar replacing of atoms (smaller atoms by larger ones) in the studied mixed crystals, the refractive index shows the quite opposite behavior. V