The photo-oxidative degradation of polyethylene/montmorillonite (PE/MMT) nanocomposite and microcomposite has been investigated. It has been found that the rate of photo-oxidative degradation of PE/MMT nanocomposite and PE/Mn+ MMT (where Mn+ stands for multivalent transition metal cation) microcomposites is much faster than that of pure PE. For the PE/MMT nanocomposite, the acceleration of photo-oxidative degradation is due to the influence of MMT and ammonium ion, in which the influence of ammonium is primary. The decomposition of ammonium ion may create acidic sites on layered silicates; meanwhile, the complex crystallographic structure and habit of clay minerals could also result in some active sites. The reversible photo-redox reaction of transition metal cations has a catalytic effect in the degradation of the polymer matrix. All these catalytic active sites can accept single electrons from donor molecules of polymer matrix and induce the formation of free radical upon UV irradiation. The generation of free radical leads to the oxidization and break of molecular chain. Thus, the materials suffer degradation and their mechanical strength decreases. (C) 2004 Wiley Periodicals, Inc.