The relationships and chemical compositions of silver sulfoselenides in the ores of the Rogovik gold-silver deposit (northeastern Russia) were studied to refine the low-temperature region of the Ag2S–Ag2Se phase diagram and identify contradictions between natural and experimental data. Two types of relationships between the phases of the system Ag2S–Ag2Se have been recognized using optical and scanning electron microscopy: (1) Se-acanthite and S-naumannite occur as monomineral microinclusions or fill cracks in the grains or the interstices of other minerals, and acanthite (free of impurities) forms rims on Fe-sphalerite; (2) Se-acanthite forms rims on S-naumannite. Electron probe microanalysis of silver sulfoselenides from the Rogovik ores revealed 0–7.9 wt.% Se in acanthite and 0–3.2 wt.% S in naumannite, which corresponds to the acanthite series Ag2S–Ag2S0.74Se0.26 and naumannite series Ag2S0.28Se0.72–Ag2Se. The composition ranges of the studied acanthite and naumannite series are wider than those of natural silver sulfoselenides from the Guanajuato (Mexico), Silver City (USA), Salida (Indonesia), and other deposits (Ag2S–Ag2S0.85Se0.15 and Ag2S0.12Se0.88–Ag2Se, respectively) but are significantly narrower than the composition ranges of synthetic samples: Ag2S–Ag2S0.4Se0.6 and Ag2S0.3Se0.7–Ag2Se. The presence of intergrowths of two phases of the Ag2S-Ag2Se series in the form of Se-acanthite rims on S-naumannite in the Rogovik ores and the absence of three-phase intergrowths of silver sulfoselenides Ag2S1-xSex from this and other deposits do not confirm the assumption on the existence of the third solid solution. The results of earlier studies of natural Ag2(S,Se) solid solutions show the existence of two solid solutions (of the acanthite and naumannite series) in the Ag2S–Ag2Se system and confirm the experimental data. It is necessary to carry out a detailed examination of natural silver sulfoselenides falling in the interval from Ag2S0.4Se0.6 to Ag2S0.3Se0.7 in order to identify the limits of two-phase immiscibility.