Molecular orbital calculations of iron, silicon, and iron silicide clusters have been carried out using the UHF-MINDO/SR method. The nature of the bonding in these compounds has been investigated by analyzing the importance of bonding indexes and diatomic components of the total energy. It has been found that in iron silicide the strongest bond is formed between Fe-Si and that it arises mainly as the result of sp-sp type orbital interactions. Although d orbitals show very little overlap with s-p orbitals, they do contribute significantly to bonding through electrostatic type diatomic interactions. By means of a detailed analysis of sp, and d orbitals and total density of states (DOS) of Fe7Si7, Si7Fe7, Fe15, and Si17 clusters, the present calculations have permitted us to explain the origin of the iron silicide UPS experimental peaks.