Zeolite supported mono- and bimetallic catalysts Pd/ZSM-5 and PdFe/ZSM-5 were prepared by exchanging Pd from a dilute aqueous solution of [Pd(NH3)(4)](NO3)(2) into either H/ZSM-5 or Fe/ZSM-5. The Fe/ZSM-5 was prepared by sublimation; it contains a major part of the Fe as dinuclear oxygen-bridged Fell ions. Significant differences between Pd/ZSM-5 and PdFe/ZSM-5 are revealed after calcination: in Pd/ZSM-5 the Pd is present as large Pd-0 particles, but in PdFe/ZSM-5 it is highly dispersed over the zeolite cavities and invisible in TEM. Evidently, the Fe ions and/or Fe oxo ions act as very effective chemical anchors. It is argued that Pd-0 is formed by autoreduction of [Pd(NH3)(4)](2+) ions even under calcination conditions. The Fe ions promote nucleation and limit growth of Pd-n(0) clusters, which are subsequently converted to PdO.