Crystals of the solid-solution series of (Ca,Sr)-lawsonite were synthesized hydrothermally at 4 GPa and 600 and 800 °C in piston-cylinder experiments. Synthesis products were analyzed with SEM, EMP, and powder-XRD. Lawsonite was observed in both the orthorhombic space group Cmcm and in the monoclinic space group P21/m. It is exclusively orthorhombic at low xSrbulk but monoclinic at high xSrbulk; in the range xSrbulk = 0.18 to 0.4 both polymorphs coexist and the data suggest a two-phase field between xSrortho ~0.1–0.2 and xSrmono ~0.3–0.4 at 4 GPa/600 °C. Linear regression to the refined lattice parameters yields a = 0.017·xSr + 5.841 (Å), b = 0.197·xSr + 8.787 (Å), c = 0.263·xSr + 13.130 (Å), and v = 4.62·xSr + 101.46 (cm3/mol) for orthorhombic lawsonite and a = 0.119·xSr + 5.306 (Å), b = 0.118·xSr + 13.160 (Å), c = 0.025·xSr + 5.833 (Å), β = 0.38·xSr + 124.07 (°), and v = 3.20·xSr + 101.59 (cm3/mol) for monoclinic lawsonite. The data suggest an increasingly negative Δvortho-mono with increasing xSr. In monoclinic lawsonite, structural expansion due to the incorporation of Sr is primarily accomplished by tilting and rotation within the Si2O7-group, whereas in orthorhombic lawsonite this tilting and rotation is prohibited by symmetry restrictions and expansion is mostly accomplished by an increase in lattice parameters. Combining the extrapolated Ca end-member volume for monoclinic lawsonite with published high-P data yields K0mono = 137(3) GPa (K′ = 4.4). Contrary to the Ca end-member system, the Cmcm–P21/m phase transition is quenchable within the Sr-bearing system. A tentative phase diagram for (Ca,Sr)-lawsonite at 600 °C indicates a narrow orthorhombic-monoclinic two-phase field that shifts significantly to lower pressure with increasing xSr. The Cmcm–P21/m phase transition in the Sr end-member system is located at ≤1 GPa at ~400 to 600 °C, 6 to 9 GPa below the transition in the Ca-system, and has a negative P-T slope.