Several new cation exchange membranes of different thicknesses (15-500 mu m) based on a Nafion(R) solution and silicotungstic acid with and without thiophene (named NASTATH and NASTA respectively) were synthesized by a simple chemical route for PEM fuel-cell applications. The optimum parameters for the preparation of the membranes have been determined: 10 mi of 5% of the Nafion(R) 117 solution was reduced by 50% and mixed with 10(-3)-10(-5) M-STA to produce a NASTA membrane. If liquid thiophene (0.5% by volume) is added to the above solution, a NASTATH membrane is produced. The water uptake and ionic conductivity of NASTA and NASTATH were compared with those of Nafion(R) 117. The effect of membrane thickness and the concentrations of STA and thiophene used during the preparation of NASTA and NASTATH on their water uptake and ionic conductivity were determined. It was shown that the water uptake of the NASTA membrane (60%) was significantly better than that of Nafion(R) 117 (27%), while the water uptake of NASTATH ( 40%) was higher than that of Nafion(R) 117 (27%). The ionic conductivity of both the NASTA (10.10 x 10(-2) Omega(-1) cm(-1)) and the NASTATH (9.5 x 10(-2) Omega(-1)cm(-1)) was found to be significantly higher than that of the Nafion(R) 117 (1.23 x 10(-2) Omega(-1) cm(-1)). The membrane performances were also determined by chemical stability studies. The membranes fabricated with Nafion(R) and silicotungstic acid with and without thiophene still exhibited good mechanical strength and stability after they had been dipped in an acid or a basic medium for at least 10 months. The voltage-current characteristics of solid polymer electrolyte fuel cells were determined for Nafion(R) 117, NASTA and NASTATH based membranes. The fuel cell parameters were correlated to the membrane water uptake and ionic conductivity. The current density at 0.600 V of the solid polymer electrolyte fuel cells (SPEFCs) based on NASTATH (810 mA cm(-2)) membranes was higher than that of SPEFCs based on Nafion(R) 117 (640 mA cm(-2)). It was shown that the better fuel cell parameters were not obtained with the modified membranes having the higher water uptake.