High pressure and temperature synthesis of compositions made of (Si1-x,Gex)O2 where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7-12 GPa and 1200-1600°C using a Kawai-type high-pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile-type phases in the SiO2-GeO2 system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si0.5,Ge0.5)O2 into SiO2-rich coesite and GeO2-rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite-stishovite phase transition pressure in SiO2.