Thermal depoling is investigated with impedance spectroscopy as a function of temperature and frequency for unmodified and ${\text{Zn}}_{\text{Sc}}^{\prime }$ modified compositions around the morphotropic phase boundary (MPB) in the high temperature material (1−x)BiScO₃–xPbTiO₃ (BSPT). By tracking the shape and width of the fundamental resonance peaks and phase angle during impedance analysis, it was shown that the depoling temperature (T_d), usually considered a single temperature, can actually be characterized as occurring over a range with an onset temperature (T_d^onset) and a final temperature (T_d^final). When this technique is applied to the rhombohedral compositions near the MPB of BSPT, it is seen that the peak narrowing in phase angle closely correlates to the rhombohedral-tetragonal phase transition decreasing from 350 °C to 300 °C when increasing from 58% to 62% PT. The T_d^onset and the subsequent correlation to the phase transition are investigated further through aliovalent substitution of 1%, 2%, and 5% Zn_Sc in BS-PT. The T_d^onset for the rhombohedral compositions displays enhancement over the unmodified compositions at ∼12 °C/1% Zn_Sc.