The gas permeation performances and various physical properties of dense polycarbonate (PC) membranes containing low molecular-weight additives (LMWA) with functional groups were evaluated. The selected LMWAs were p -nitroaniline, 4-amino 3-nitro phenol, catechol and 2-hydroxy 5-methyl aniline, and LMWA concentration in the membrane was changed between 1 and 10% (w/w). The PC/LMWA membranes had higher ideal selectivities but lower permeabilities than pure PC membrane. The most effective LMWA which provided the highest selectivity and the lowest permeability in membranes was p -nitroaniline and the least effective one was catechol. The selected LMWAs antiplasticized the PC matrix and improved the separation performances even at low concentrations. The glass transition temperature (T-g) of PC/LMWA membranes was lower than that of pure PC membrane. Reduction of T(g)s as a function of p -nitroaniline concentration obeyed to Gordon-Taylor model which indicates antiplasticization type interaction. FTIR spectra of pure and blend membranes also demonstrated the existence of interaction.