Cross-linked sulfonated poly(imide-siloxane) (cSPI-SX) is obtained by radical polymerization onto the silylmethyl group of poly(dimethylsiloxane) (PDMS). The chemical structure of the cSPI-SX thus formed is characterized by FT-IR and C-13 NMR. The properties required for a proton exchange membrane, such as ion exchange capacity, water uptake, dimensional change, hydrolytic stability, and proton conductivity, are measured and investigated in detail. A transmission electron microscopic (TEM) image shows the presence of large, well-connected hydrophilic domains that are responsible for the high proton conductivity of the cSPI-SX membrane. The unique phase separated morphology of the cSPI-SX membrane supports the outstanding hydrolytic stability. A PEFC assembled with the cSPI-SX membrane has showed high power output at a humidifier gas temperature of 30 degrees C, a cell operating temperature of 70 degrees C and a gas pressure of 0.3 MPa. (c) 2012 Elsevier B.V. All rights reserved.