This paper concerns heterogeneous single site catalysts which are difficult to prepare because of speciation problems related to the two starting components, the solid and the metal precursor solution. A two-step post-synthesis method is used to prepare the V-SiBEA reference system, with vanadium in framework position. In the light of the molecular processes identified for each step of the post-synthesis method, other transition elements are selected and investigated. The pros and cons of the method are discussed. While redox zeolites with isolated and framework metal centres have been successfully obtained, the results raise the question as to which extent the latter have to be identical, from a chemical and/or topological point of view.