Thermal decomposition in a thermobalance of Fe(CO)5 adsorbed on alkali-metal, hydrogen-Y, dealuminated Y, L and omega zeolites proceeds stepwise via slow decarbonylation at low and high temperatures, separated by a fast endothermic reaction. Average CO/Fe ratios have been determined after each step. From i.r. results the former intermediates are assigned to species bearing bridging CO, whereas reaction products with CO/Fe < 1 are associated with highly unsaturated carbonyl clusters in strong interaction with the zeolite.The thermal stability of zeolite/Fe(CO)5 adducts as well as of the intermediates increases with the electron-donor properties of the matrix and can be rationalized using the Sanderson electronegativity concept. Iron loadings ranging from 2.4 wt % in zeolite L up to 10 wt % with NaY and HY are obtained by decomposition in inert atmosphere. Under vacuum conditions loss of metal up to 50% is observed. Metallic iron clusters are the final decomposition products in alkali-metal zeolites, as probed by NO adsorption. In HY part of the metallic iron is oxidized to FeII ions, which are located at cation positions.