Y-doped BaZrO3 (BZY) can be deposited epitaxially on MgO (0 01) and the considered interface serves as a model system for studying heterointerface properties of protonic conductors. In this study, the defect chemistry of the interface between ZrO2-terminated BaZrO3 (0 01) and MgO (0 01) was investigated by first-principles calculations and space-charge theory. Segregation energies from the BZY bulk to the interface ZrO2 and MgO layers were calculated for effectively charged protons, oxygen vacancies, Y-acceptors as well as cation vacancies. Protons were found to exhibit a strong tendency for segregating to the interface, particularly to an oxide ion in the MgO layer, rendering a net positive charge of the interface. According to the applied thermodynamic space charge models, the interface potential could reach more than 1 V under the Mott-Schottky approximation, with depletion regions extending up to 2 nm into BZY. With fully equilibrated Y-segregation profiles, the interface potential was significantly diminished to about 0.2 Vat 573 K and 0.025 bar H2O. While the interface was found to be close to saturated by protons under most condition, it was concluded that proton conduction along the interface could not contribute significantly to the in-plane conductivity of BZY films deposited on MgO substrate. (C) 2016 Elsevier B.V. All rights reserved.