The Brunauer-Emmett-Teller (BET) method has been used extensively to characterize the surface areas of porous materials by semiempirical fitting of gas-adsorption isotherms. However, questions often arise concerning the applicability and the exact meaning of the BET areas. In particular, there has been much debate about whether the BET method provides a faithful description of the geometrical areas of porous materials if the atomic structures are exactly known. In this work, we provide a comprehensive analysis of the BET areas for both model slit pores and crystalline porous materials using the grand canonical Monte Carlo simulation. Based on extensive simulation data for nitrogen adsorption at 77 K and the conventional models of materials characterization, we find no simple correlation between the BET and geometrical surface areas. For materials with the same BET area, their geometric surface areas may vary over one order of magnitude. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 286-293, 2018