The effect of the structural properties and the oxidation state of MN on the 18O isotope exchange behaviour of ternary manganites (La1-xSrxMnO3, La0.5Sr1.5MnO4 and SrMnO3) has been studied. All types of 18O isotope exchange (homomolecular, partially and completely heteromolecular) take place on the very active manganites with perovskite (LaMnO3 and La0.7Sr0.3MnO3) and perovskite-like (SrMnO3) structure, but not on the less active K2NiF4-structure (La0.5Sr1.5MnO4). The highest 18O exchange activity is observed for La0.7Sr0.3MnO3, for which the completely heteromolecular 18O exchange starts to occur at 520 K, already, a Ton which is typical for excellent redox catalysts. The influence of the structural properties on the 18O exchange and oxygen diffusion behaviour of the manganites is much more pronounced than that of the Mn3+/Mn4+ ratio. The different reduction behaviour of the manganites with perovskite and K2NiF4-structure can be explained by means of the bond-valence model.