Calcium–strontium solid-solutions of margarite, anorthite, slawsonite, calcite and strontianite were synthesized from oxide–hydroxide–fluid mixtures by way of hydrothermal experiments at 400–500 °C and 390–500 MPa. The fractionation of Ca and Sr between the coexisting phases was investigated via electron microprobe and X-ray diffraction analyses of the solids and inductively coupled plasma–optical emission spectrometry of the fluids. A complete solid-solution series of margarite is indicated and the lattice parameters a, b, c, and V increase linearly with increasing Sr content. The lattice parameters of (Ca,Sr)-anorthite show a likewise linear increase with increasing Sr content. The formation of Sr–Ca slawsonite and Sr–Ca strontianite is restricted to high bulk Sr contents. The fractionation of Ca and Sr between minerals and fluid expressed as KD(Sr–Ca)mineral–fluid increases in the following sequence: calcite–fluid < anorthite–fluid ≤ margarite–fluid ≈ 1 ≤ strontianite–fluid < slawsonite–fluid. At low XSr (i.e., for common metamorphic bulk-rock compositions), KD(Sr–Ca)calcite–fluid = 0.09, KD(Sr–Ca)anorthite–fluid = 0.51, and KD(Sr–Ca)margarite–fluid = 0.56; the Sr/Ca ratio of the fluid is therefore generally higher than that of the coexisting minerals. This suggests that during prograde metamorphism and probably continuous dehydration, the Sr/Ca ratio of the rock decreases continuously with time. But, owing to the roughly equal Sr–Ca mineral–fluid fractionation for margarite and anorthite, which was also determined for zoisite/clinozoisite and lawsonite, this decrease will result in only moderate changes of whole-rock and fluid Sr-composition as long as these phases are the main Sr-bearing minerals.