Epitaxial CeO
2
films with different thickness were grown on Y
2
O
3
stabilised Zirconia substrates. Reduction of cerium ions at the interface between CeO
2
films and yttria stabilised zirconia substrates is demonstrated using aberration-corrected scanning transmission electron microscopy combined with electron energy-loss spectroscopy. It is revealed that most of the Ce ions were reduced from Ce
4+
to Ce
3+
at the interface region with a decay of several nanometers. Several possibilities of charge compensations are discussed. Irrespective of the details, such local non-stoichiometries are crucial not only for understanding charge transport in such hetero-structures but also for understanding ceria catalytic properties.