Polyisobutylene (PIB), an important elastomer with a low glass transition temperature, presents markedly low permeability properties to small-molecule penetrants compared to other elastomers. In the past, computer simulation approaches to explain this behavior have led to diffusivity and solubility calculations that, unfortunately, deviated significantly from the experimental values. We present here the results of a new simulation strategy which leads to accurate predictions of the solubility of four gases (He, At, N-2, and O-2) in PIB, thus opening up the way toward understanding the molecular origin of the superior barrier properties of PIB. A critical element in the new approach is the introduction of a reliable united-atom model for PIB that can accurately reproduce its conformational characteristics and volumetric properties over a wide range of temperature conditions, thereby providing well-equilibrated representative PIB structures for subsequent permeability studies with a more accurate force field. To this, independent PIB configurations thoroughly pre-equilibrated with the new model are converted to all-atom PIB structures, re-equilibrated using the detailed COMPASS force field, and employed in a series of sorption runs for the estimation of the infinite dilution solubility coefficient, So, of the small gas molecules. Simulation results for the solubility of He, Ar, N-2, and O-2 in PIB at room temperature are found to reproduce experimental data with very good accuracy. Additional results at progressively higher temperatures show that the solubility of O-2 and Ar is always higher than that of N-2 and He, respectively. We also find that calculations based on a united-atom representation overestimate systematically the solubility of these gases, with the exception of He.