Abstract This chapter deals with the isomorphous substitution in the zeolitic framework. Some specific cases among the research works of the authors have been selected. B-MFI, Fe-MFI, Fe-BEA and Fe-MOR zeolites have been examined optimizing the synthesis parameters. B-MFI zeolite has been synthesized in fluoride media, while the other zeolites have been prepared in alkaline media. In all cases isomorphous substitution in the zeolitic structure, even if not complete, is obtained. It should be pointed out that in fluoride media and in presence of K+ and Cs+ ions more than four boron atoms per unit cell can be incorporated in the MFI structure. To cite this article: R. Aiello et al., C. R. Chimie 8 (2005).