A sulfonated poly(ether ether ketone) (SPEEK) membrane with a fairly high degree of sulfonation (DS) can swell excessively and even dissolve at high temperature. To solve these problems insolvable functionalized silica powder with sulfonic acid groups (SiOx-S) was added into the SPEEK matrix (DS = 55.1%) to prepare SPEEK/SiOx-S composite membranes. The decrease in both the swelling degree and the methanol permeability of the membranes was a dose-dependent result of addition of the SiOx-S powder. Pure SPEEK membrane swelled 52.6% at 80 degrees C, whereas the SPEEK/SiOx-S (15%, by mass) membrane swelled only 27.3% at the same temperature. From room temperature to 80 degrees C, all SPEEK/SPEEK/SiOx-S composite membranes had methanol permeability of about one order of magnitude lower than that of Nafion (R) 115. Compared with pure SPEEK membranes, the addition of the SiOx-S powder not only leads to higher proton conductivity, but also increases the dimensional stability at higher temperatures,and greater proton conductivity can be achieved at higher temperature. The SPEEK/SiOx-S (20%, by mass) membrane could withstand temperature up to 145 degrees C, at which in 100% relative humidity (RH) its proton conductivity exceeded slightly that of Nafion (R) 115 membrane and reached 0.17 S.cm(-1), while pure SPEEK membrane dissolved at 90 degrees C. The SPEEK/SiOx-S composite membranes are promising for use in direct methanol fuel cells because of their good dimensional stability, high proton conductivity, and low methanol permeability.