Cesium hydrogen salt of heteropolyacids (CsHPs) including Cs(2.5)H(0.5)PMo(12)O(40) (CsPMo) and Cs(2.5)H(0.5)PMo(12)O(40) (CsPW) are incorporated into Nafion to produce stable nanocomposite membranes at the moderate temperature/low relative humidity (RH). The addition of hygroscopic and conductive CsHP enhances the water content but limits the activity of the sulfonic group of the Nafion. As a result, ion exchange capacity (IEC) is decreased. The increase in conductivity versus RH is higher. The conductivity at anhydrous and high-temperature condition is higher due to the additional water retention or additional surface functional sites. The results of oxidative stability of membranes show that the CsPW/Nafion composite membrane has superior stability against oxidative agents due to the CsPW in lowering H(2)O(2) diffusion. The nanocomposite membranes have better performance in the PEM fuel cell test at temperatures 60, 80 and 100 degrees C (35% RH, ambient pressure) than plain Nation membrane. The stability of single cells under a constant load demonstrates that the decay rate for plain Nation membrane is rapid due to the dehydration. The covering effect for the CsPW particle is stronger than the CsPMo particle results in higher water uptake, IEC, conductivity and fuel cell performance and lower voltage decay for the CsPMo/Nafion membrane rather than the CsPW/Nafion membrane. (C) 2011 Elsevier B.V. All rights reserved.