Antimony/magnesium/oxygen clusters are produced by a gas aggregation source, in which a mixture of antimony and magnesium is vaporized and reacted with N2O introduced in helium carrier gas. The resulting product distribution is detected by a time-of-flight mass spectrometer following ionization with a KrF excimer laser. Four types of cluster products are observed : Sb-x(+), SbxMgyOz+, SbxMgy+, and MgyOz+. The mass spectral intensity distributions display enhanced abundances for Mg2O+, Sb2-4Mg3O+, Sb1-4Mg2O+, Sb4Mg+, Sb5Mg2+, and Sb6Mg2+. The experimental observation of Mg2O+ and Mg3O+ shows that the suboxides of group 2 are stable species, consistent with theoretical predictions. The binding abilities of antimony clusters to magnesium and magnesium oxides are found to be dependent on cluster size. When the number of antimony atoms in the clusters is smaller than 6, SbxMgyOz+ are the main products dominating the mass distribution. On the other hand, when the cluster size of Sb-x is larger than 6, only Sb/Mg alloy clusters are observed. The unusual stabilities of Sb2-4Mg3O+ and Sb1-4Mg2O+ clusters are evidently due to the formation of covalent bonds between Sb and Mg atoms. In Sb4Mg+, Sb5Mg2+, and Sb6Mg2+ alloy clusters, however, the Mg atom donates two electrons to the skeleton of the Sb clusters in order to satisfy Wade’s rules. The structures of these stable clusters can be predicted by the polyhedral skeletal electronic pair theory.