The incorporation of additives into polymers like bisphenol A polysulfone (PSF) can provide an attractive alternative for modifying their permselective properties for gas separation, provided the additive is appropriately selected. In this work, three series of additives based on naphthalene, bisphenol A, and fluorene structures were incorporated into PSF and their effect on gas transport properties was measured. The results show that these additives increase selectivity and reduce permeability of PSF when incorporated at low concentrations. The largest increases in selectivity for the gas pairs He/CH4, H-2/CH4, and O-2/N-2 are caused by those additives that cause the largest reductions in the glass transition temperature and fractional free volume, and typically, they are made up of planar molecules containing polar or small asymmetric protuberances. The productivity-selectivity balance response of the modified PSF membranes reveals that the naphthalene-based additives containing asymmetric groups of atoms lead to membranes with higher selectivity and with only small losses in permeability relative to the unmodified PSF membranes. The addition of 20 wt % of the glassy additive Kenflex A into PSF causes a fourfold increase in the selectivity of PSF to He/CH, at the expense of a 37% reduction in the permeability of helium.