Intermetallic compounds are unique catalyst platforms for mechanistic studies and industrial applications, because of their ordered structures in comparison to random alloys. Despite the intrinsically defined stoichiometry of intermetallic compounds, compositional deviations can still occur in intermetallic catalysts. The location of the extra metal atoms could differ the catalytic properties of intermetallic compounds with non-stoichiometric composition if those metal atoms end up on/near the surface. In this study, we synthesized PtSn intermetallic compounds with accurate stoichiometry and slightly Pt-/Sn-rich compositions. We used furfural hydrogenation and acetylene semi-hydrogenation as probe reactions to investigate the surface structures of PtSn intermetallic catalysts after reduction at different temperatures. Even though the intermetallic PtSn is the major bulk phase among non-stoichiometric compositions, the intermetallic PtSn surface can only be observed under the high-temperature reduction in Sn-rich PtSn intermetallic nanoparticles (iNPs), while the Pt-rich PtSn iNPs show Pt-rich-surfaces regardless of reduction temperatures. Four structural models were constructed based on the comprehensive surface and bulk characterizations. This work extends the understanding of intermetallic catalysts with nonstoichiometric compositions to tailor the intermetallic surface structures for catalysis. (C) 2019 Elsevier Inc. All rights reserved.