Polymer films with aligned impermeable flakes can have permeabilities 10- 100 times smaller than films of the same dimensions but without flakes. However, if the flakes have a distribution of sizes, this reduced permeability could be compromised by the shorter diffusion paths in any region containing smaller flakes. This work shows theoretically and experimentally that this is not the case. For example, a film with 3 vol.% of 5 mum and 3 vol.% of 50 mum flakes has a diffusional resistance 20% higher than a film with 6 vol.% of only 5 mum flakes and only 1% less than a film with 6 vol.% 50 mum flakes. (C) 2004 Elsevier B.V. All rights reserved.