A synthetic feldspar of composition Ca0.2Sr0.8Al2Si2O8 (An20SrF80) (space group I1̅ at room condition, Qod = 0.88) was investigated using in situ high-pressure single-crystal X-ray diffraction. Unit-cell parameters were measured as a function of pressure to 7.7 GPa at room T. Two reversible phase transitions were observed in the investigated pressure range. A first-order triclinic I1̅ to monoclinic I2/c phase transition was observed at P ~ 4.3 GPa. Several cycles through the transformation were made to constrain the hysteresis (~0.3 GPa). The discontinuous character of the transition is especially noticeable in the behavior of the γ angle, which increases from 90.55° to 91.04° before Ptr. The Murnaghan EoS parameters obtained for the volume are: V0 = 1393.97(6) Å3, K0 = 88.7(5) GPa, K′ = 1.3(3) for the triclinic I1̅ phase and V0 = 1396.9(1) Å3, K0 = 89.4(9) GPa, K′ = −1.7(6) for the monoclinic I2/c phase whose high-P behavior appears, therefore, linear in the pressure range of its stability. The axial compressibility scheme is βa > βc > βb for both phases, in good agreement with previous observations for other feldspars. A similar displacive ferroelastic I1̅-I2/c phase transition was observed for the same sample with increasing temperature. At P ~ 7.3 GPa a large discontinuity in the unit-cell parameters indicates a further transition from monoclinic I2/c phase to monoclinic-II phase. This transformation is strongly fist order with a volume change of about 1.7%.