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The solubility of Au and Cu in hydrous andesite melts was determined at 1000°C and 200MPa. The data show that the solubility of Au is highly influenced by reduced S (S²⁻), and to a lesser degree, by the Cl concentration in the melt. Other melt compositional parameters such as K₂O/Na₂O ratio, or FeO concentration have no significant effect. The simultaneous presence of S²⁻ and high concentrations of Cl (0.7–1.3wt.%) yield Au solubilities higher than expected from separate contributions of Au-sulfide and Au-chloride species. The strong effect of S and Cl on Au solubilities suggests dissolution mechanisms similar to those observed in magmatic volatiles. The most likely dominant dissolved species are AuSH or AuS(K/Na) in sulfur-bearing, and AuCl in Cl-bearing, S-free systems. At the S²⁻/S⁶⁺ transition (∼NNO+0.2), the solubility of Au sharply drops by a factor of 5. Copper shows markedly different behavior …