The incorporation of low molecular mass additives to polyetherimide (Ultem(R) 1000) led to the polymer's antiplasticisation, an effect indicated by a reduction of its free volume, an increase of its gamma sub-T-g (T-gamma) transition temperature and storage modulus, and a reduction of the sorbed carbonic gas level and the gas permeability coefficient. The extent of these changes, however, was found to be strongly dependent on a combination of additive properties, i.e. size, free volume, degree of interaction with the polymer. and the glass transition temperature.Based on a theoretical correlation between the diffusion coefficient and the segmental motion frequency, we argue that the increase in T,. which indicates intensified segmental restriction, is related to a reduction of the permeability coefficient. However. the real influence of this transition on the permeability could not be analysed because the additives also change other parameters that influence gas permeability. We assume that several of these parameters are considered in the mixtures' fractional free volume (FFV), since a relatively good correlation was found between this parameter and the permeability. The scatterings found in this correlation were attributed to alterations in segmental motions and free volume distributions of PEI, which are not considered by the FFV parameter but that affect the gas permeability coefficients. (C) 2003 Elsevier Science B.V. All rights reserved.