Partial detemplation of zeolites followed by desilication in alkaline meduim is demonstrated as a powerful and elegant approach to design hierarchical zeolites with tailored degree of mesoporosity. This achievement, illustrated for large beta crystals, is based on the fact that the template-containing zeolite is virtually inert to Si leaching upon treatment in aqueous NaOH solutions. Partial removal of the structure-directing agent creates regions in the crystal susceptible to mesopore formation by subsequent desilication, while template-containing regions are protected from silicon extraction. Variation of the calcination temperature in the range 230-550 degrees C determines the amount of template removed and enables control of the extent of mesopore formation in the zeolite (20-230 m(2) g(-1)) upon alkaline treatment. The functionality of the introduced mesoporosity in the hierarchical beta crystals is demonstrated by the improved performance in the catalytic pyrolysis of low-density polyethylene. The partial detemplation-desilication treatment enhances the tuning options of this demetallation method.