Silver nanoparlicles of a relatively narrow Size distribution were prepared in the structural cavities of the commercia] ionomer membrane thin film Nafion. UV-vis absorption, X-ray powder diffraction, and transmission electron microscopy methods were used in the characterization of the nanoparticle-Nafion composite thin films. The population of silver nanoparticles in the membrane structure could be varied by changing the preparation conditions. but the sizes of the nanoparticles were found to remain generally similar. The results were explained in terms of the hosting of the silver nanoparticles in homogeneously distributed structural domains in the membrane structure, where each domain is probably a group of interconnected reverse micelle-like ion clusters according to the ion cluster model for ionomer membranes already in the literature. (C) 2004 Elsevier B.V. All rights reserved.