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Here, we report results from hydrothermal alteration and crystallization experiments at 800 °C and 1 GPa that constrain the partitioning of S, Fe, Sr, and Ce (as a proxy for the REE) between fluorapatite and hydrothermal fluids of variable compositions. The data from these experiments demonstrate that S can be incorporated in apatite by the two known coupled substitutions S6+ + Na+ = P5+ + Ca2+ and S6+ + Si4+ = 2P5+. The data also demonstrate that the presence of Sr in hydrothermal fluids promotes the incorporation of Na and S in apatite via the coupled substitution S6+ + Na+ = P5+ + Sr2+. Our data also reveal a previously unknown intrinsic relationship between Fe and S, and Ce and S in metasomatized fluorapatite that can be explained by coupled substitutions Ca2+ + P5+ = Fe3+ + S4+ and Ca2+ + P5+ = Ce3+ + S4+, respectively. The concentrations of Cl, OH, and S in run-product apatite are positively correlated with each other, which indicates that the presence of Cl and OH can play a determinative role in the incorporation of S in apatite. Overall, the data from these metasomatism experiments involving S and apatite demonstrate that incorporation of sulfate and sulfite into apatite during metasomatism depends on the abundance of charge-balancing cations in the fluid.