Oxy-chalcogenide glasses with compositions of xGeO(2)-(100 - x)GeS2, where 0 <= x <= 100 mol%, have been prepared and studied in terms of their structures and optical properties. X-ray fluorescence spectroscopy shows that Ge:S ratio can deviate from GeS2 by similar to 10 at.%, depending critically upon the preparation conditions. Raman scattering spectroscopy suggests that stoichiometric GeO2-GeS2 glasses have a heterogeneous structure in the scale of 1-100 nm. The optical gaps are nearly constant at 3.0-3.5 eV for glasses with 0 <= x <= 80 mol% and abruptly increase to similar to 6 eV in GeO2. This dependence suggests that the optical gap is governed by GeS2 clusters, which are isolated and/or percolated. Composition-deviated glasses appear as orange and brown, and these glasses seem to have more inhomogeneous structures. (c) 2007 Elsevier B.V. All rights reserved.