Several commercially available samples consisting of agglomerated small mordenite crystallites with low Si/Al ratios (5.7–10at/at) have been treated in aqueous NaOH solution. It was found that the porosity can be enhanced when the sodium hydroxide solution is sufficiently concentrated. Treatment in 1M NaOH for 15min resulted in inter-crystalline porosity and the mesopore volume was increased from 0.01 to 0.21cm 3 g −1 together with an increased external surface area from 36 to 85m 2 g −1 . The micropore volume and crystallinity were preserved after the treatment. Both H-MOR and Na-MOR mordenite agglomerates have been successfully treated: the Na-MOR requires a longer contact time to obtain similar porosity. By carefully choosing the alkaline concentration and contact time, intra-crystalline mesoporosity can be obtained for mordenite with Si/Al ratios as low as 10. Catalytic tests with proton mordenite showed that alkaline treatment leads to more than one order of magnitude of activity gain in the liquid-phase alkylation of benzene with propene to form cumene, while selectivity is preserved. These results demonstrate that alkaline treatment also on high-aluminum content mordenites is an effective tool to enhance accessibility and thereby its catalytic performance.