Sol-gel reactions of tetraethylorthosilicate have been performed in three different zinc-neutralized ethylene-methacrylic acid copolymer ionomers for the purpose of creating nanostructured organic-inorganic materials with enhanced properties. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to understand the effect of this in situ reaction on ionic aggregate morphology. EXAFS spectra showed that no large changes occurred in the aggregate structure, indicating that the internal environment of the aggregates did not nucleate the reaction, at least to the extent that the ionic domains were significantly undisrupted. However, small changes in EXAFS patterns were evident. These changes could be a result of nucleation of the reaction at the edges of the aggregates, or aggregate size rearrangement caused by the insertion of inorganic moieties into the polymer. However, the results indicate that the latter cannot include significant amounts of unaggregated zinc ions. (C) 2000 John Wiley & Sons, Inc.