Ultraviolet-curable nanocomposites containing organically modified nanoclays were prepared to serve as barrier coatings against oxygen and water permeation. A novel in situ synthesis technique was used to produce well-dispersed clays in an unsaturated polyester polymer before crosslinking. The in situ dispersion route was compared with nanocomposites prepared by mixing and sonication for several levels of nanoclay loading (1, 2, 5, and 10 wt %). The comparison of nanocomposite properties prepared from each processing method demonstrated that the in situ preparation technique led to better clay dispersion as verified by transmission electron microscopy. The in situ route for nanoclay dispersion produced nanocomposites with lower water vapor transmission and permeability compared with the sonicated dispersion method. The impact on cure characteristics, mechanical properties, thermal stability, and optical clarity of the nanocomposites were also compared. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012