Crystallized zirconia nanoparticles with diameters of 6.3 +/- 0.5 nm were in situ formed in Nafion solution through sol-gel process. Nafion molecules were self-assembled onto zirconia nanoparticles through electrostatic interactions and prevent the further growth of initial formed particles. The Nafion-zirconia nanocomposite membranes were formed using a recasting process. It was found that the addition of zirconia nanoparticles did not affect the crystallinity and structure of Nafion in the membrane significantly. The formed Nafion-zirconia nanocomposite membrane shows enhanced water retention ability at 100 degrees C compared to recast pure Nafion membrane, especially at medium and high relative humidity. This work demonstrates the potential of Nafion-zirconia nanocomposite membranes for PEMFC applications. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.