The BET analysis is commonly used for determining surface areas of metal-organic Frameworks (MOFs) and zeolites that contain "ultra-micropores" (< 7 angstrom) even though it is often stated that the BET surface areas obtained for such small pores are not really meaningful in an absolute sense. In this study, nitrogen and argon isotherms in MOFs and zeolites (most of them having ultra-micropores) were predicted by grand canonical Monte Carlo (GCMC) simulations and used as pseudoexperimental data to evaluate the BET method for these structures. The BET surface areas calculated from the simulated nitrogen and argon isotherms agree well with Me accessible surface areas obtained directly from crystal structures in a geometric fashion. However, this was only true when the BET analysis was performed using the appropriate pressure range based on published "consistency criteria"; the BET analysis underestimates the surface areas of all the selected MOFs and zeolites if it is done in the "standard" BET pressure range. Moreover, the BET theory was shown to work well for a zeolite with a highly heterogeneous surface. These results validate the application of the BET method for determining surface areas of MOFs and zeolites even for materials having ultra-micropores or heterogeneous pores.