We have studied the interface properties of nominally undoped and of donor (La) doped Pb(Zr,Sn,Ti)O₃ (P(L)ZST) by means of X-ray photoelectron spectroscopy. In order to avoid charging of the highly insulating materials during photoemission, thin conducting electrodes, which are permeable for the photoelectrons, have been applied to the surface of the materials. The comparison between high work function RuO₂ and low work function Sn-doped In₂O₃ (ITO) electrodes reveals a difference of the Fermi level position at the interface of 1.1 eV. This is similar to the difference in work function of the two electrode materials, indicating the absence of Fermi level pinning. The same behavior is observed for ferroelectric and anti-ferroelectric compositions. We have further studied the interface behavior at elevated temperature with applied electric field across the sample. Undoped PZST exhibits a substantial increase in current with time (resistance degradation). In contrast, the conductivity of donor-doped PLZST, which is almost 3 orders of magnitude lower than that of undoped PZST, decreases with time. Despite the strongly different electrical behavior, both samples develop a metallic Pb component at the interface with increasing voltage stress. These experiments constitute a unique opportunity to study the chemical stability of interfaces of highly insulating materials.