  The class of antiperovskite compounds $A_3B$O ($A$ = Ca, Sr, Ba; $B$ = Sn,
Pb) has attracted interest as a candidate 3D Dirac system with topological
surface states protected by crystal symmetry. A key factor underlying the rich
electronic structure of $A_3B$O is the unusual valence state of $B$, i.e., a
formal oxidation state of $-4$. Practically, it is not obvious whether anionic
$B$ can be stabilized in thin films, due to its unusual chemistry, as well as
the polar surface of $A_3B$O, which may render the growth-front surface
unstable. We report X-ray photoelectron spectroscopy (XPS) measurements of
single-crystalline films of Sr$_3$SnO and Sr$_3$PbO grown by molecular beam
epitaxy (MBE). We observe shifts in the core-level binding energies that
originate from anionic Sn and Pb, consistent with density functional theory
(DFT) calculations. Near the surface, we observe additional signatures of
neutral or cationic Sn and Pb, which may point to an electronic or atomic
reconstruction with possible impact on putative topological surface states.
