The aim of this work was to study the effects of incorporation of low molar mass additives on the molecular mobility and water vapor transport properties of the polysulfone (PSF). The additives used in this work were N-phenyl-2-naphthylamine (PNA) at 10, 18, and 30 wt% concentration and 2,6-di-terc-butyl p-cresol (BHT) at 5, 10, 15, and 20 wt% concentration. The additive incorporation resulted in changes on molecular mobility and thermal properties of the polysulfone glassy matrix associated with antiplasticization phenomenon. The effects observed on the polysulfone were reduction in glass transition temperature, reduction in the magnitude of secondary loss transition peak, changes in secondary loss transition peak for higher temperatures, and increase in elastic modulus E' as compared with those of the unmodified polymer. Changes in molecular mobility were correlated to reductions in PSF water vapor permeability. In PSF-PNA mixtures, the water vapor permeability was reduced up to 95% for 30 wt% additive incorporation and 81% for mixtures PSF-BHT with 20 wt% incorporation. (c) 2006 Wiley Periodicals, Inc.