The thermal decomposition of cesium-silver-gold(III)-chloride Cs2Ag1-xAu1+x:/3Cl6 with 0 <x ≤ 0.63 at temperatures between 250 and 350 °C yields, under evolution of chlorine, mixed crystals of Cs2AgAuCl6 and Cs2AuIAuIIICl6. The composition of the latter can be described by the formula Cs2Ag1-y,AuyIAuIIICl6 with y = 2x;/ (3-x). Apart from the formation of the mixed crystals the reaction leads to CsCl as a further phase, and in the case of x < 0.35 also to some Cs2AgAuCl6. Mixed crystals with 0.23 ≤ y < 1, free from additional solid phases, can be obtained from strong hydrochloric acid solutions of Cs2AgAuCl6 and Cs2AuIAuIIICl6, which contain an excess of cesium ions. The mixed crystals are isostructural with Cs2AuIAuIIICl6 (space group I4/mmm). The variation of the lattice constants with increasing silver content confirms that the univalent silver, in contrast to the univalent gold, shows no tendency toward the formation of linear dichloro complexes. Our investigations show that the "tetragonal modification of Cs2AgAuCl6", described by Elliott and Pauling and obtained by them by heat treatment of cubic cesium-silver - gold(III)-chloride, is, in fact, CS2Ag1-y/AuyIAuIIICl6.