Mordenite (MOR) membranes are membrane is one of the water permselective membranes that can remove water vapor from water/alcohol mixtures. A permeation mechanism through non-zeolitic pores (intercrystalline pathways) of MOR zeolite membranes was discussed using an alkaline treatment of the MOR membranes. The alkaline treatment was carried out at 70 degrees C in 0.2 M of NaOH solution. Permeation properties were evaluated by single gas permeation of N-2, water/isopropyl alcohol (IPA) pervaporation (PV), and perm-porometer measurements. Between 0 and 30 min of the alkaline treatments, the IPA permeances were kept constant and the water permeances decreased with the length of the treatment periods, showing that the MOR membrane was densified by the treatment. Between 30 and 40 min of the treatments, N2 and IPA permeances increased with increasing the treatment periods while water permeance were kept constant. The non-zeolitic pores were formed by the alkaline treatment. The water selectivity decreased after 40 min of the treatment. The size of the non-zeolitic pores was 0.9 nm after 62 min of the treatment. The water/IPA selectivity through the membranes decreased to 94 from 2800 because of the alkaline treatment. Thus, we concluded that the size of the water selective non-zeolitic pores is less than 0.9 nm.