An Ag2S/Ag heteronanostructure has been prepared for the first time by hydrochemical deposition. The “acanthite -Ag2S–argentite -Ag2S” phase transformation has been studied in situ by high-temperature X-ray diffraction and transmission electron microscopy. The crystal structure of argentite has been revealed. It has been found that the concentration of vacant sites in the metal sublattice of argentite exceeds 92%. The reversible acanthite–argentite transformation in the Ag2S/Ag heteronanostructure at the application of the external bias voltage is considered.