The formation of positively charged antimony oxide clusters has been investigated as a function of oxygen partial pressure using time of flight mass spectrometry. With increasing oxygen partial pressure magic number patterns are observed, which can be attributed to the clusters of (Sb2O3)n+ and (Sb2O3)n(SbO)+ with 2⩽n<7 and 1⩽n<7, respectively. Oxygen rich clusters, i.e., clusters formed by the addition of one or more oxygen atoms to the above observed series, gain in intensity with increasing cluster size and increasing oxygen partial pressure. To obtain information about structures and general building principles of these clusters we have performed ab initio quantum chemistry calculations for the series (Sb2O3)n, (Sb2O3)n+, (Sb2O3)n(SbO)+, (Sb2O3)n(SbO2)+, and (Sb2O3)n(SbO3)+. Except from a defect center in the cationic series, antimony atoms are trivalent and oxygen atoms divalent. For the cationic series (Sb2O3)n(SbO)+, an open valence at one antimony atom is avoided by the formation of an additional Lewis-donor Lewis-acceptor Sb–O bond with a trivalent oxygen atom. Open structures with terminal oxygen atoms or with Sb–Sb bonds are very high in energy, expressing a principle of the formation of a maximum number of Sb–O bonds. In the series (Sb2O3)n(SbO2)+, an excess oxygen valence leads to structures with a central Sb+VO4 unit (i.e., without terminal oxygen). For n⩾3, such structures have a very pronounced energetic stability compared to isomers with a terminal oxygen atom or with an O–O bond. Characteristic building blocks in the neutral and in the cationic series are eight-membered rings, which are also found in the bulk antimony(III)oxide modification senarmonite, as well as Sb4O5 units bridged by oxygen atoms.