The perovskite solid solution (1−x)BiScO3–xPbTiO3 (BSPT) represents a new family of piezoelectric ceramics that are promising for high-temperature actuator applications. In the current work, BSPT and PZT ceramics were synthesized by the conventional mixed oxide method. Thermal degradation and aging at 250°C for 1000 h for undoped BSPT with morphotropic phase boundary (MPB) x=0.64 and tetragonal x=0.66 compositions are compared with pure PZT with the MPB composition. The BSPT system showed reduced aging rates of the piezoelectric coefficient (2.1% and 0.8%/decade for MPB and tetragonal compositions, respectively), compared with the unmodified PZT ceramics (4.2%/decade). The Rayleigh measurement approach was used to conclude that in both the BSPT and PZT, the dominant aging mechanism was the suppression of the domain wall contributions associated with charge defect accumulation at domain walls. The large thermal stability of the poled domain state and low domain wall contributions yielded the lowest observed aging rates in the tetragonal BSPT samples suggesting that this material would be a strong candidate for high-temperature actuator applications above 200°C.