Correlation between transport properties and free volume has been widely evidenced by several authors. We are showing through a positron annihilation study in a high barrier polymer that transport properties can be improved by means of antiplasticizers addition. We also are verifying in this paper the influence of both free volume hole size and free fractional volume in diffusion of gases, finding a relationship with solubility too. (c) 2006 Elsevier B.V. All rights reserved.