Although the research presented in this thesis is performed in the inorganic chemistry and catalysis group at the Eindhoven University of Technology, this thesis does not contain any chemical (catalytic) reactions. Instead, it deals with a subject of physical chemistry, the diffusion of molecules in zeolites. At first sight, these two subjects do not seem to be too closely related. Diffusion can however be a very important parameter when understanding the catalytic performance of chemical reactors, something which will hopefully become clear in the next few sections.