For the first time, the a-Ag2S (acanthite)–b-Ag2S (argentite) phase transformation in nanocrystalline and coarse-crystalline powders of silver sulfide has been observed in situ by the scanning electron microscopy method in real-time. The argentite crystals are formed on the surface of acanthite particles as a result of electron-beam heating. According to the differential thermal analysis data, the transformation occurs at a temperature of B449–450 K, and the enthalpy of transformation is equal to B3.7–3.9 kJ mol1 . The presence of a-Ag2S (acanthite)–b-Ag2S (argentite) phase transformation is confirmed in situ by high-temperature X-ray diffraction data.