The low-energy structures of PtnSnn (n = 1–10) and Pt3mSnm (m = 1–5) clusters have been determined using genetic algorithm incorporated with density functional theory. Platinum and tin atoms tend to mix with each other due to the energetically favorable Pt–Sn bonds. However, due to the larger atomic radius of Sn atoms, we find segregation of Sn atoms on the surface of PtnSnn clusters. This leaves one or two Pt atoms available for reaction and for larger clusters segregation of Sn could block the Pt sites. For Pt3mSnm clusters, Sn atoms are well separated in the cluster structures and prefer to form sharp vertices leaving triangular faces of three Pt atoms available for reactivity. The electronic properties such as highest occupied molecular orbital–lowest unoccupied molecular orbital gap, distribution of frontier orbitals, Mayer bond order, Mülliken atomic charge, and the density of states are discussed. Significant hybridization between the d orbitals of Pt and the p orbitals of Sn is revealed. These theoretical results provide the general trends for the structural and bonding characteristics of the Pt–Sn alloy clusters and help understand their catalytic behavior. © 2014, Springer Science+Business Media New York.