Colorless, transparent SiO2-substituted GeO2 single-crystals with the alpha-quartz-like structure have been grown by spontaneous nucleation using the slow-cooling method from K2O-3MoO(3) flux. The cell parameter values as well as the semi-quantitative chemical analyses confirm the presence of Si-content in the as-grown crystals. Their thermal behavior was studied by means of differential scanning calorimetry and high-temperature X-ray diffraction. The high stability of the alpha-quartz-like structure as a function of temperature was confirmed up to 1000 degrees C. The structural parameter data have been used to estimate the lattice and volume thermal expansion coefficients from room temperature up to 1000 degrees C. (C) 2011 Elsevier B.V. All rights reserved.