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Andesite melts were equilibrated with an H–O–S-bearing volatile phase to determine the partition coefficients for S and Cl as a function of melt composition and oxygen fugacity. The experiments were conducted in rapid-quench MHC vessel assemblies at 200MPa and 1000°C, and over a range of imposed fO₂ between NNO−1.2 and NNO+1.8. High fluid/melt mass ratios (∼15) were employed, allowing precise and accurate partition coefficients to be obtained by mass balance calculations. Chlorine exhibits Henrian behavior at ClO_−0.5 activities typical for arc magmas, with D_Cl^{volatile/melt}=1.36±0.06 (1σ) below 0.2wt.% Cl in the melt; at higher ClO_−0.5 activities, D_Cl^{volatile/melt} increases linearly to 2.11±0.02 at 1wt.% Cl in the melt. In the volatile phase: FeCl₂∼NaCl>KCl∼HCl. The determination of cation exchange coefficients for major cations yielded: K_K,Na^{volatile/melt}=1.23±0.10 (1σ) and ^∗K_Fe,Na^{volatile/melt} …