High pressure and temperature synthesis of compositions made of (Si1-x,Ge-x)O-2 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 degrees C using a Kawai-type high-pressure apparatus. At 12 GPa and 1600 degrees 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 degrees C, but not at 1200 degrees C. At lower pressures (7 and 9 GPa) and 1600 degrees C, we observed the decomposition of (Si-0.5,Ge-0.5)O-2 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.