Zeolites and (ordered) mesoporous materials are essential building blocks of functional materials used in, for example, adsorption, separation, and catalysis. [1] Key factors governing the performance of these materials are pore size, pore shape, surface properties, and the like. In many applications, especially for zeolites, the molecules involved either as substrate or product have similar sizes to those of the pores. [2] This leads to molecular-sieving properties relevant for adsorption and separation processes and shape selectivity in catalysis. Likewise, the rate of access of molecules into zeolite crystals may be low or sometimes even zero if pore blocking by impurities occurs. To improve the accessibility of zeolites many different approaches have been followed, such as the synthesis of zeolites with large pores, [3][4][5][6][7] small crystals, [8] hierarchical structures, [9] and mesoporous crystals. [1,10,11] In all cases, however, one clearly needs reliable methods for the characterization of the accessibility of these materials. Adsorption studies, [12] often in combination with spectroscopy, [13][14][15][16] have been used until now to establish the average accessibility of the micropore volume of zeolite samples. Progress has been made in recent years in obtaining information on the accessibility of porous materials with techniques such as magnetic resonance imaging (MRI) [17] or interference microscopy [18] with a spatial resolution of millimeters and micrometers, respectively. Herein, we use adsorption and diffusion studies in combination with scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis, which allow us to determine quantitatively the [*] Dr.