Despite many recent attempts to unravel the structure of novel PdZn alloys, which are prontising catalysts in methanol steam reforming, a detailed study on the formation of Pd-Zn alloy particles and of their structural and thermal stability is still missing. We take advantage of the unique properties of epitaxially grown Pd particles embedded in layers of amorphous ZnO and mechanically stabilized by SiO2, and present an electron microscopy study of the preparation of well-ordered PdZn alloy nanoparticles at surprisingly low reduction temperatures. These are formed by topotactic growth on the surface of the I'd nanoparticles and are structurally and thermally stable in a broad temperature range (473-873 K). At and above 873 K, partial decomposition of PdZn and the start of interaction with the SiO2 support are observed. (c) 2006 Elsevier Inc. All rights reserved.