In recent years, polymeric membranes have been increasingly used in key unit operations such as gas separations. In theory, the performance of a polymeric membrane module depends solely on the chemical structure of the polymer and the feed gas composition. It has, however, been observed that impurities in the feed stream (such as vapors from compressor oils) affect the productivity of the membrane module even at parts per million (ppm) levels. In order to understand the fundamental effects of such trace chemicals, commercial polymers were studied for their solubility and permeability characteristics in the presence and absence of such impurities. Solubilities of N-2, O-2, CH4 and CO2 were measured in dense homogeneous films at 35 degreesC. Two types of compressor oils were used in doping the polymers at three concentration levels. Results from the solubility data were well described by the dual-mode model for glassy polymers that consists of a Henry's solubility term k(D), Langmuir sorption capacity C'(H), and an affinity constant "b." The effect of the oil on each of the model parameters is presented. The effect of annealing on sorption in clean and doped samples of the polymers was also examined. Such thermal treatment affected the Langmuir sorption capacity Cf, more than any of the other dual-mode parameters. Annealing lowered C'(H) because of the ensuing densification of the glassy structure. This effect was common to both clean and doped films. A good understanding of the effects of such trace chemicals on gas solubility in synthetic membranes will assist in examining effects of impurities on the overall membrane productivity and aid in designing effective polymeric membrane modules in the future.