Zirconium phosphate as inorganic compound was chosen for investigations concerning mainly the swelling behaviour of composite membranes for the direct methanol fuel cell (DMFC). Swelling in liquid systems and in vapour systems at 100% relative humidity conditions was investigated. The fluxes of water and methanol through the membranes were obtained from pervaporation experiments. The conductivity of the developed membranes was determined by impedance spectroscopy. Two different cells for impedance measurements were used. In one cell, the membrane sample is in contact with an electrolyte solution during the measurement. In the second cell, swelling of the membrane sample can be varied by controlling temperature and relative humidity (RH). The in situ generation of inorganic oxides like zirconia by hydrolysis of the alkoxides in the polymer solution leads to a decrease of water and methanol flux through the membranes. The addition of well-dispersed zirconium phosphate to the polymer solution increases the membranes' conductivity. Both effects can be explained by the swelling behaviour of the composites. The performance of some membranes in a methanol fuel cell test system is discussed with regard to the swelling behaviour and the methanol permeability. (C) 2003 Elsevier B.V. All rights reserved.