The molecular characteristics of air sorption and the subsequent separation of oxygen and nitrogen by dense glassy polymer membranes have been studied in detail by large-scale molecular dynamics (MD) simulations. Air was inserted at various pressures into explicit reservoirs on either side of an similar to 50000-atom 6FDA-6FpDA polyimide thin film. The simultaneous sorption of oxygen and nitrogen and their transport through the dense polymer were followed for more than 300 ns. This very long time scale (with respect to current fully atomistic MD simulations) allowed for the concentration of both penetrants to reach equilibrium in the matrix. When exposed to air, the thin film undergoes a very limited amount of swelling. Oxygen is more soluble than nitrogen in the polyimide even if, following sorption, both penetrants tend to occupy the same range of favorable low-energy sites. These sites represent less than 2.5% of the matrix core volume for O-2 and less than 2% for N-2. Such subtle differences lead to a model solubility selectivity of similar to 1.5, which is in excellent agreement with experiment. The equilibrium uptake curves for both penetrants are pressure-dependent, but their N-2/O-2 concentration ratio is systematically similar to 2.7 degrees Compared to the value of similar to 3.7 for pure air, the gas within the thin film is thus oxygen-enriched air (OEA). In terms of mobility, both O-2 and N-2 molecules exhibit complex trajectories with oscillations within the available matrix low-energy sites combined with occasional jumps to different sites due to temporary channels created by the fluctuations of the polyimide chains. O-2 can cross the thin film almost twice as fast as N-2, thus improving the efficiency of the separation. Indeed, the relative number of penetrants having crossed the membrane after 300 ns is similar to 60% O-2 and similar to 40% N-2. The average gas diffusion coefficients in the inner part of the model film and the corresponding diffusion selectivity of similar to 3.4 are also in very good agreement with experiment. This leads to a model mixed gas separation factor of similar to 5 for O-2/N-2 in 6FDA-6FpDA, as compared to 4.7 for experiments. In addition to its ability to complement experiments, this suggests that, with the continuous growth of available computer resources state-of-the-art MD could be efficiently used as a prescreening tool for such separation applications before undertaking complex syntheses and membrane developments. This could prove especially useful when structures are closely related (e.g., isomers) and their simulations parameters are similar.