Our general aim is to understand the defect structure of complex oxides in ternary oxide systems, which are known to be effective selective oxidation catalysts. For this purpose we will investigate in detail, (a) iron molybdate and (b) bismuth molybdates. which are industrially used for selective oxidation of hydrocarbon molecules. These materials are prepared by a variety of methods, in particular through solid-state reaction or through co-precipitation followed by calcinations at specific temperatures. More recently, we developed low-temperature hydrothermal methods to produce these solids, in crystalline form. Therefore, it is our aim is to determine the structural differences, between the three methods of preparation, as prepared, after catalytic reaction with hydrocarbon to determine the loss of structural oxygen and after re-oxidation.