Optical, electron microprobe, and x-ray diffraction analysis of 88 samples of various compositions between Ag2S and Ag2Se synthesized at high temperature in sealed quartz tubing indicates the presence of two solid-solution series in this system at ambient (room) conditions. One series extends from Ag2S to approximately Ag2S0.4Se0.7 and has the Ag2S-III-type structure (monoclinic). The second series ranges from Ag2S0.3Se0.7 to Ag2Se and is characterized by the Ag2Se-II-type structure (orthorhombic). Members of both series, in appropriate proportions, characterize the apparent compositional gap between the two solid solutions. Gradual shifts in the locations of the x-ray diffraction peaks along the compositional gradient of each solid solution revealed an expansion of the d-spacing as the larger Se ion was substituted for S in the Ag2S-III-type structure and a contraction as S was substituted for Se in the Ag2Se-II-type structure. The reported discrete phase, Ag4SSe (aguilarite, orthorhombic), appears to be simply a member of the monoclinic Ag2S-III-type solid solution.