(Ca,Sr)-walstromite with the mean composition Ca0.43Sr0.57[SiO3] was synthesized at 4 GPa/600 °C and XSrbulk= 0.875 in presence of a 1 molar aqueous (Ca,Sr)Cl2 fluid together with monoclinic (Ca,Sr)-lawsonite, grossular and strontianite. Intracrystalline as well as walstromite–mineral and walstromite–fluid Ca–Sr distribution was determined by analysing the product fluid with ICP–OES for Ca and Sr concentrations, solids by electron-microprobe analysis, powder XRD-analysis with Rietveld refinement and single-crystal X-ray diffraction. Based on the single-crystal X-ray diffraction data the crystal structure of (Ca,Sr)-walstromite was solved in space group P-1 with refined lattice parameters a = 6.7580(9) Å, b = 9.464(3) Å, c = 6.7507(16) Å, α = 83.22(2)°, β = 76.83(2)°, γ = 70.33(2)°, and V = 395.46(17) Å3. The data indicate that (Ca,Sr)-walstromite has slightly smaller volume and A-sites than the polymorph (Ca,Sr)-wollastonite-II over the entire compositional range. Coordination numbers and mean bond lengths for the three A-sites in Ca0.43Sr0.57-walstromite are <[VIII]A1–O>= 2.588 Å, <[VI]A2–O>= 2.369 Å, and <[VII]A3–O>= 2.660 Å. The smaller volume suggests that (Ca,Sr)-walstromite is the high-P/low-T polymorph. The data show extreme intracrystalline fractionation of Sr within the (Ca,Sr)-walstromite solid solution series with exchange coefficients KD(Sr – Ca)site1 – site2 defined as (Sr/Ca)site1/(Sr/Ca)site2 of KD(Sr–Ca)A3–A1 = 21, KD(Sr–Ca)A3–A2 = 2400, and KD(Sr–Ca)A1–A2 = 114. The walstromite–mineral and walstromite–fluid Ca–Sr distribution indicates notable fractionation of Sr into the coexisting phases lawsonite, strontianite and fluid compared to (Ca,Sr)-walstromite. Grossular has only very minor amounts of Sr. Exchange coefficients KD(Sr–Ca)phase1–phase2= [(Sr/Ca)phase1]n/[(Sr/Ca)phase2]m with n and m being the stoichiometric coefficients of the respective exchange reaction are 0.00068 for walstromite–fluid, 0.000011 for walstromite–lawsonite and walstromite–strontianite, and 31.8 for walstromite–grossular.