Our experimental study of the system AgAuS-AgAuSe was urged by the misfit in literature data on the isomorphism of sulfur and selenium in this series and on the existence of AgAuSe phase. Scanning electron microscopy, optical microscopy methods, electron microprobe analysis and X-ray powder diffraction method have been applied to study samples synthesized by heating stoichiometric mixtures of initial compositions AgAuS1-xSex (x = 0; 0.25; 0.5; 0.75; 0.875; 1). Results of the studies evidence the existence of continuous solid solution of the AgAuS-AgAuSe series. As a result of the partial isomorphous replacement Se → S, the unit cell parameters of petrovskaite-like type (R3¯m) phases are slightly increased compared to AgAuS. The phases AgAuS1-xSex at 0.5 < x ≤ 1 are, most likely, metastable, their instability drastically increasing with the growing content of selenium. The phases partly decompose to form a fischesserite-like phase Au3Au(Se,S)2 or fischesserite (Au3Au(Se,S)2) (at x = 1), high-fineness Au-Ag alloy, S,Se phase or Se (at x = 1) and β-(Au,Ag) (Se,S) (at x = 0.875 and 1). Gold selenide AuSe showed isomorphism Se → S to 0.17 mol fraction of sulfur and Au → Ag - to 0.1 mol fraction of silver. The content of AgAuSe phase, synthesized under different heating, annealing and cooling conditions, was the highest in the experiment with rapid cooling from 1050 °C without annealing. The existence of phase AgAuSe in natural systems is likely to be very problematic.