The transport properties of hybrid membranes with sulfonated poly(ether ether ketone) (sPEEK) as polymeric matrix (sulfonation degree, SD, of 71 and 87%) are studied as a function of the membrane thickness and the content of zirconium oxide (2.5, 5.0 and 7.5 wt.%), incorporated via in situ zirconia formation. Nafion((R)) 112, 1135 and 117 are used as reference materials. The investigated properties of the composite membranes are the proton conductivity, water uptake, methanol and water permeation fluxes and water/methanol selectivity. Impedance measurements indicate that proton conductivity decreases when thickness increases for the WEEK composite membrane with SD of 87%; the same effect is observed for Nafion((R)). In contrast, it was found that the membrane thickness has almost no effect on the proton conductivity for sPEEK with SD = 71%. In terms of water uptake, it was found that it slightly increases with the thickness for all the sPEEK membranes tested, while the effect on Nafion((R)) is far more evident. For most of the studied membranes, methanol and water permeation fluxes, as well as selectivity towards water/methanol, decrease with the membrane thickness. Finally, it was found that the zirconium oxide incorporation in the sPEEK polymer leads to membranes with decreased proton conductivity, water swelling and water and methanol permeation fluxes. However, it increases the composite membranes selectivity towards water/methanol. (c) 2006 Elsevier B.V. All rights reserved.