(T)ransition metal ions in zeolites TMI-zeolite (TM = Fe, Co, Cu) attract great attention due to their potentialities as catalysts. In the recent years, the high efficiency of TMI-zeolites for the selective catalytic reduction (SCR) of contaminated flue gases has been demonstrated. It has been shown that the structure of the framework, the nature and location of extraframework cation species play a fundamental role in the process. Experimental results based on spectroscopies, as well as on reactivity studies have led to valuable insights about the structure of cationic sites as well as about the active species involved during the catalytic reactions. However, it is not sufficient to obtain all this information. This review reporting density functional theory (DFT) calculations shows that a molecular approach is very useful and has become an indispensable tool for the determination of the geometries, the electronic structures, the spectroscopic properties and the reactivity of TMI-zeolites. (c) 2005 Elsevier B.V. All rights reserved.