We have investigated the molecular sieving mechanism in carbon molecular sieves (CMSs), by a combination of molecular dynamics simulations and pore network theory, using the separation of air into oxygen and nitrogen as a prototype. Molecular dynamics simulations of diffusion in individual pores show that the diffusion coefficients for oxygen and nitrogen are strong functions of pore width and that the degree of kinetic separation (expressed as the ratio of the species diffusion coefficients) observed experimentally can be reproduced at the level of individual pores. A critical path analysis of diffusion in the pore network of the CMS shows that this degree of kinetic separation can also be obtained with pore networks having a range of pore sizes, even where the width of the pore size distribution is large.