"In situ" high temperature X-ray diffraction under reducing atmosphere is used for the first time to study the thermal stability and transformations of Co- and Fe-exchanged A and X zeolites TG-DTA and "ex situ" XRD characterization were also carried out The temperature of incipient crystallization of metallic phase was found to be 700 degrees C in Fe-zeolites and 800 degrees C in Co-zeolites Moreover, ex situ X-ray experiments, after thermal treatment both under inert and reducing atmosphere, revealed the formation of ceramic phases upon the thermal collapse of the zeolitic framework Metal nanoparticles were obtained by reduction and the size of metal clusters was found to range between 24 and 40 nm (C) 2010 Elsevier Ltd All rights reserved