Diffusion in nanoporous host–guest systems is often considered to be too complicated to comply with such “simple” relationships as Fick´s first and second law of diffusion. How ever, it is shown herein that the microscopic techniques of diffusion measurement, notably the pulsed field gradient (PFG) technique of NMR
spectroscop yand microimaging by interference microscopy (IFM) and IR microscopy (IRM), provide direct experimental evidence of the applicability of Fick´s laws to such systems. This remains true in many situations, even when the detailed mechanism is complex. The limitations of the diffusion model are also discussed with reference to the extensive literature on this subject.