This paper presents an evaluation of the zirconium oxide effects in sulfonated poly(ether ether ketone) (sPEEK) with sulfonation degree (SD) of 87%. A series of inorganic-organic hybrid membranes were prepared with a systematic variation of the zirconium oxide content via in situ zirconia formation (2.5, 5.0, 7.5, 10, 12.5 wt.%). This procedure enabled the preparation of proton electrolyte membranes (PEM) with a wide range of properties, which can be useful for evaluating the relationship between the PEM properties and the direct methanol fuel cell (DMFC) performance. The investigated properties are the proton conductivity, proton transport resistance, water uptake, water, methanol, oxygen, carbon dioxide and nitrogen permeability coefficients, morphology and elemental analysis. The results obtained show that the inorganic oxide network decreases the proton conductivity and water swelling. It is found that it leads also to a decrease of the water, methanol, carbon dioxide and oxygen permeability coefficients, an increase of the water/methanol selectivity and a decrease of the carbon dioxide/nitrogen and oxygen/nitrogen selectivities. In terms of morphology, it is found that in situ zirconium alkoxide hydrolysis enables the preparation of homogeneous membranes that present a good adhesion between inorganic domains and the polymer matrix. (C) 2004 Elsevier B.V. All rights reserved.