Compositional dependence of lattice parameters and unit cell volumes of TlMeX2-type (Me = Ga or In and X = S or Se) layered mixed crystals with monoclinic structure has been studied by X-ray diffraction technique. It is revealed that the lattice parameters of TlGa1-xInxSe2, TlGa(S1-xSex)(2), TlGa1-xInxS2 and TlIn(Se1-xSx)(2) mixed crystals increase due to the substitution of gallium atoms by indium and sulfur atoms by selenium. Moreover, the absorption edges of these mixed crystals have been investigated through the transmission and reflection measurements in the wavelength range of 400-1,100 nm. The analysis of absorption data has revealed the presence of both optical indirect and direct transitions. It is found that the energy band gaps of mixed crystals decrease due to the substitution of gallium atoms by indium and of sulfur atoms by selenium.