In this work a new route for preparation of core-shell nanoparticles composed of an iron oxide core and a polypyrrole (PPy) shell is explored. During the preparation procedure the initially formed iron(0) core is converted to magnetite. It is demonstrated, that the magnetite cores can completely be dissolved by reaction with acid. Furthermore the dissolution of iron oxide cores by electrolysis also is possible. The resulting PPy hollow spheres as well as the core-shell nanocomposites are electrochemically active. (C) 2010 Elsevier Ltd. All rights reserved.