Mixtures of an epoxy resin and organophilic montmorillonites were subjected to ultraviolet (UV)-induced photopolymerization. Two types of commercially available nanoclays, namely Cloisite 30B and Cloisite Na+, were modified through interaction with organic compatibilizers (dodecylsuccinic anhydride, octadecylamine, octadecyl alcohol, and octadecanoic acid). The modified nanoclays, dispersed in the liquid epoxy resin at 5 wt%, were photopolymerized to get nanocomposite films. The kinetics of the photopolymerization was evaluated by means of real-time Fourier transform infrared spectroscopy. The modified nanoclays and their nanocomposites were characterized through X-ray diffractometry; transmission electron microscopy showed the presence of intercalated and partially exfoliated morphologies in the nanocomposites. Thermogravimetric and dynamic-mechanical analyses showed an increase of the thermal properties and an increase of the glass transition temperatures of the nanocomposites compared with that of the neat UV-cured resin. Finally, the oxygen barrier properties of nanocomposite films, coated on a polyethyleneterephtalate substrate, were evaluated; the decrease of permeability was correlated with the degree of exfoliation of the nanocomposites. POLYM. ENG. SCI., 50:14001407, 2010. (C) 2010 Society of Plastics Engineers