The permeability of polymer films can be reduced dramatically with many layers of thin mineral flakes aligned parallel to the film's surface. For a film with a volume fraction φ of flakes of aspect ratio α, the permeability reduction is expected to be proportional to αφ for the dilute limit (φ &MLT; 1 and αφ < 1) but proportional to (αφ)(2) in the semi-dilute limit (φ &MLT; 1 but αφ > 1). Permeabilities of hydrochloric acid and sodium hydroxide across films containing mica or vermiculite in polyvinyl alcohol agree with the second, semi-dilute prediction. Permeabilities of helium across films of montmorillonite in polyethylene glycol do as well. These improvements in barrier properties are independent of flake size, permeant and polymer chemistry. © 0 2005 Elsevier B.V. All rights reserved.