Composite membranes formed of zeolitic fillers embedded in Naf ion can be made by evaporating the solvents from a suspension of small zeolite crystals in a Nafion solution. Two natural zeolites were selected as fillers: chabazite and clinoptilolite. Membranes with various zeolite content were obtained. Composite membranes with zeolite content up to 40 vol.% exhibited uniform distribution of the zeolite fillers throughout the thickness, as ascertained by scanning electron microscopy (SEM), energy dispersive X-rays absorption (EDX) and ion conductivity measurements. Although more brittle than Nafion, these composite sheets retain-when hydrated -reasonable flexibility below 40 vol.% zeolite content. We have determined the hydrogen ion conductivity and the permeability of methanol molecules through these membranes (in the H+-form as well as in the Na+-form) in the temperature range 22-60 degreesC. We found that the presence of the zeolitic fillers in the membranes can bear notable changes of conductivity, permeability and selectivity with respect to membranes made out of sole Nafion. (C) 2003 Elsevier Science Ltd. All rights reserved.