Alloys of the gold-cobalt system characterized by limited solubility have been synthesized by the high-pressure torsion method. According to the data of X-ray diffraction analysis it was revealed that as a result of mechanical alloying, a phase of a supersaturated solid solution based on gold is formed in the composition of the alloy. Lowering the temperature to cryogenic one leads to a redistribution of dissolved cobalt and an increase in its concentration in the solid solution. The solid solution phase is in an ultrafine-grained state. Possible mechanisms for the formation of a supersaturated solid solution in the gold-cobalt system are discussed taking into account the specific features of the severe plastic deformation and mechanical alloying processes.