Sorption separation of nitrogen-oxygen mixtures by carbon molecular sieves, e.g. to produce nitrogen using the pressure swing adsorption principle, proceeds under the condition of non-equilibrium. Experimental data based upon measurements of sorption equilibria and kinetics for nitrogen and oxygen on several samples of carbon molecular sieve particles in the temperature range 0-50-degrees-C suggest the existence of a surface barrier in the pore mouths of slit-like micropores for these particular samples. The conclusion on this phenomenon is supported by the results of canonical and grand canonical Monte Carlo simulations. Both types of analysis give a deeper insight into the complex non-equilibrium separation process.