Shedding light on the nature of surface barriers of nanoporous materials, molecular simulations (Monte Carlo, Reactive Flux) have been employed to investigate the tracer-exchange characteristics of hydrocarbons in defect-free single-crystal zeolite membranes. The concept of a critical membrane thickness as a quantitative measure of surface barriers is shown to be appropriate and advantageous. Nanopore smoothness, framework density, and thermodynamic state of the fluid phase have been identified as the most important influencing variables of surface barriers. Despite the ideal character of the adsorbent, our simulation results dearly support current experimental findings on MOF Zn(tbip) where a larger number of crystal defects caused exceptionally strong surface barriers. Most significantly, our study predicts that the ideal crystal structure without any such defects will already be a critical aspect of experimental analysis and process design in many cases of the upcoming class of extremely thin and highly oriented nanoporous membranes.