In order to increase the stability of Nafion conductivity at temperatures higher than 100 degrees C, composite membranes made of recast Nafion filled with different percentages of zirconium phosphate (ZrP) were investigated. The membrane preparation was carried out by a simple synthetic procedure based on the use of solutions of ZrP precursors in dimethylformamide. The formation of insoluble alpha-type ZrP nanoparticles within the Nafion matrix was proved by P-31-MAS NMR and X-ray diffractometry. The membranes were characterized by TEM microscopy, ion-exchange capacity determinations, static stress-strain mechanical tests and conductivity measurements as a function of filler loading, at controlled relative humidity (r.h.) and temperature. An increasing filler loading results in enhanced membrane stiffness and in lower conductivity compared with pure recast Nafion. At 90% r.h. and 100 degrees C, the conductivity decreases from approximate to 0.07 S cm(-1) for pure Nation to approximate to 0.03 S cm(-1) for the composite membrane containing 25 wt.% ZrP. Systematic conductivity measurements as a function of r.h. and temperature were carried out to draw a stability map for the conductivity of pure recast Nafion and of a composite membrane filled with 10 wt.% ZrP. These maps provide for each r.h. value the maximum temperature at which the conductivity remains stable for at least 150 h. The effect of zirconium phosphate is to increase the stability of conductivity at high temperature, with a gain up to 20 degrees C. This stability enhancement has been ascribed to the higher stiffness of the composite membrane. (C) 2007 Elsevier Ltd. All rights reserved.