New hybrid membranes containing polyvinyl alcohol (PVA) and poly(tetramethylene oxide) (PTMO) with heteropolyacid (HPA) as a hydrophilic inorganic modifier in an organic/inorganic matrix were developed for low-temperature proton exchange membrane fuel cells (PEMFCs). A maximum conductivity of 4.8 x 10(-3) S cm(-1) was obtained at 80 A degrees C and 75% RH for PVA/PWA/PTMO/H(3)PO(4) (10/15/70/5 wt%), whereas the PVA/SiWA/MPTS/H(3)PO(4) (50/10/10/30 wt%) membrane demonstrated a maximum conductivity of 8.5 x 10(-3) S cm(-1) under identical conditions. These hybrid composite membranes were subsequently tested in a fuel cell. A maximum current density of 240 mA cm(-2) was produced at 70 A degrees C for the PVA/PWA/PTMO/H(3)PO(4) membrane, and the corresponding value for the PVA/SiWA/MPTS/H(3)PO(4) membrane under identical conditions was 230 mA cm(-2). The small deviations in cell performance can be explained in terms of the variations in thickness of the membranes as well as differences in their conductivities. The fuel cell performances of these membranes decreased drastically when the temperature was increased to 100 A degrees C.