Structural modifications induced by the I1̅-I2/c displacive transition in Ca0.2Sr0.8Al2Si2O8 feldspar (An20SrF80) have been investigated in situ by single-crystal X-ray diffraction at 20, 200, 400, 500, and 620 °C. Crystals were synthesized from the melt, cooled slowly to 1300 °C, and then quenched in air. At room temperature (a = 8.361, b = 12.973, c = 14.259 Å, α = 90.79, β = 115.55, γ = 90.62°, V = 1394.9 Å3; space group: I1̅; Qod = 0.88), the polyhedra of the non-tetrahedral cation have different configurations at the Ca/Sr(0) and Ca/Sr(z) sites. In monoclinic Sr feldspar, the Sr-OB and Sr-OD distances are regular, but in triclinic An20SrF80 feldspar the OB(m0) atom in the Ca/Sr(0)-polyhedron and the OD(mz) atom in the Ca/Sr(z)-polyhedron are displaced. The topochemical symmetry of the frame-work is essentially monoclinic and the average dimensions of the pseudo-symmetrical tetrahedra do not change within the error limits. With increasing temperature, the distances between the Ca/Sr cations and the pseudo-related O atom pairs converge on the values adopted at the I1̅-I2/c transition. At the transition point, the M-polyhedron assumes a regular coordination, similar to that observed in monoclinic Sr feldspar at room temperature. The results obtained indicate Ttr = 520 ± 10 °C, in agreement with the transition temperature obtained from the changes of cell dimensions. The variation of cos2α* with temperature is consistent with the solution to Landau 2-4-6 potential with Tc = 506 ± 7 °C.