A thin film coating with tailorable thickness and clay concentration was prepared by solution casting an aqueous slurry containing poly(vinyl alcohol) (PVOH) and montmorillonite (MMT) clay. The anisotropic clay platelets have excellent alignment due to self-orientation during drying, which results in good transparency and oxygen barrier. A 50 wt % clay coating, with a thickness around 4 mu m and visible light transmission of 58%, improves the oxygen barrier of a 179 mu m poly(ethylene terephthalate) (PET) substrate by more than 3 orders of magnitude. This PVOH/MMT composite thin film also has good thermal stability and mechanical properties. This simple coating procedure could be used for a variety of packaging applications that use plastic film (e.g., food, pharmaceutical, and electronics).