The Ca-Sr fractionation between zoisite and, respectively, lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl 2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients K D (Sr-Ca) A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are K (Sr-Ca) zo A1-af = 3.38 to 41.08 for the A1 site and K (Sr-Ca) zo A2-af = 0.45 to 6.51 for the A2 site. Assuming Ca af = Sr af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca 2+(af) + Sr A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + Ca A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] and Ca 2+(af) + (M 2+ ) A1 Sr A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + (M 2+ ) A1 Ca A2 Al 3 [Si 3 O 11 (O/OH)] yields μ 0 = –29 kJ/mol and W Sr-Ca zo A1 = 5.5 kJ/mol for the A1 site and μ 0 = –1.1 kJ/mol and W Sr-Ca zo A2 = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P 2 1 /m form. Calculated lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K D (Sr-Ca) law Cmcm -af = 1.12 to 11.32 for orthorhombic and K D (Sr-Ca) law P 21 m -af = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca 2+(af) + SrAl 2 Si 2 O 7 (OH) 2 ·H 2 O = Sr 2+(af) + CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O assuming Ca af = Sr af and a symmetric mixing model yields similar values of μ 0 = –9 kJ/mol and W Sr-Ca law Cmcm = 10 kJ/mol for orthorhombic and μ 0 = –10 kJ/mol and W Sr-Ca law P 21 /m = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are D Sr zo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D Sr law Cmcm -af = 0.6 ± 0.2 for orthorhombic lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and lawsonite-bearing rocks. For low Sr-concentrations, x Sr zo directly reflects x Sr af and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.