The possibilities of heterosegregation for generating surface phases are investigated by synthesizing the GaN phase on a GaAs single-crystal surface. The new phase is created for the first time on a gallium arsenide surface as a result of autosegregation during which gaseous nitrogen or argon with nitrogen and hydrogen admixtures is pumped through the appearing layer of liquid gallium at temperatures of 970–1050°C. The nanomorphology and elemental and phase compositions of the GaN surface layer formed by surface reactions are characterized via high-resolution scanning electron microscopy, optical digital microscopy, and X-ray spectrum microanalysis.