The aim of this work was to study the effects of the addition of low molar mass additives such as N-phenyl-2-naphthylamine (PNA) and hexafluoro-bisphenol-A (HFBPA) on the volumetric, thermal and transport properties of polyethersulfone (PES). The additive incorporation resulted in changes in molecular mobility and properties of the glassy PES matrix associated with antiplasticization. The changes in molecular mobility were ac-companied by reductions in water vapor permeability of the PES of up to 93% for 30 wt % incorporation of PNA and of up to 70% for the addition of 20 wt % of HFBPA. (c) 2006 Wiley Periodicals, Inc.