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The main goal of the work described in this thesis is to obtain (meso-) porous zeolites by post synthesis treatment for improved catalytic performance, with a focus on mordenite and cumene synthesis. The zeolite samples consist of small crystallites in the range between 20 and 200 nm that have agglomerated into larger particles of ~0.5 to 2.0 m. It is shown that alkaline or the combination of acid and alkaline treatment results in porous mordenite, with intra- as well as inter-crystalline mesopores present. Electron tomography reveals that mostly intra-crystalline mesopores are formed that are in contact with the external surface, thereby providing fast mass transfer to and from the active sites. Catalytic tests for the production of cumene shows that the activities of these porous mordenites are close to the activity of a commercial catalyst based on zeolite Beta, with reduced selectivity towards the formation of undesirable n-propylbenzene. Alkaline treatment on various template containing zeolites, resulted in purely intercrystalline mesopores and increased external surface areas, independent of the Si/Al ratio. The added porosity was obtained without changing the Brnsted acid sites because of the protection of the template molecules. This enables us to decouple acidity and accessibility of zeolites.