Single crystals of Ge(Se1-xSx)(2) with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 were grown by vertical Bridgman method. The crystalline phase and stochiometry of the layered crystals were investigated by X-ray, SEM and EPMA techniques. The energy band gaps of the whole series crystals were examined by thermo reflectance (TR)and transmission measurements. Compositional dependences of the band gap and broadening parameter for Ge(Se1-xSx)(2) were evaluated. The relationship for the composition-dependent direct band gaps of Ge(Se1-xSx)(2) is determined to be E-g(x) = (2.43 +/- 0.03) + (0.38 +/- 0.02)x + (0.26 +/- 0.02) x(2)eV. (c) 2005 Elsevier B.V. All rights reserved.