The effect of the particle size and specific surface area of silver sulfide powders on their density measured by the helium pycnometry method has been studied. Powders with different average particle sizes have been synthesized by hydrochemical deposition. The particle size of the silver sulfide powders has been determined by the Brunauer–Emmett–Teller method according to molecular nitrogen adsorption isotherms and, in the case of fine powders, by X-ray diffraction analysis according to diffraction reflection broadening. It has been shown that the density of the fine powders measured by helium pycnometry is underestimated compared with the true density, owing to the adsorption of helium by the highly developed surface of these powders.