Transport of molecules through microporous crystals can be influenced by transport limitations at the mouths of pores. In the limit of high temperatures, the kinetics of adsorption into micropores is dominated by steric effects. To assess the role of molecular shape and pore size on pore mouth adsorption kinetics, discontinuous molecular dynamics has been used to simulate a variety of hard-sphere molecules adsorbing into slit, circular, and elliptical pores. Particular attention has been given to pores with length scales similar to zeolites. By simulating a range of straight chain, branched, and cyclic molecules, the selectivities that can arise due to differences in adsorption kinetics as functions of molecular size and shape have been probed.