A thermodynamic description of the intermetallic compounds in the Zr–Sn binary system has been obtained using total energy calculations by means of the Vienna ab initio simulation package. Our calculations show that hexagonal compounds Zr 5 Sn 4 and Zr 5 Sn 3 are the most stable phases in the Zr–Sn binary system. Their high stability is found to be due to hybridization of the Sn 5p with Zr 4d electronic states. Based on the calculated energies, the conclusion is made that Zr substitution on the Sn sites takes place in the Zr 4 Sn phase, which accounts for the unusual stoichiometry of this Cr 3 Si structure type compound.