Nafion 117™-SnO 2 composite membranes were prepared by in-situ particle formation using an ion-exchange method. SnO 2 was incorporated into Nafion 117™ membranes by ion-exchange in solutions of SnCl 2 . 2 H 2 O in methanol, followed by oxidation to SnO 2 in air. By adjustment of the concentration of SnCl 2 ∙ 2 H 2 O used in the ion-exchange step, compositions ranging from 2 to 8wt.% SnO 2 with SnO 2 homogeneously distributed as nanoparticles were obtained.The prepared nanocomposite membranes were characterized by powder XRD, 119 Sn MAS NMR spectroscopy, electrochemical impedance spectroscopy, water uptake, tensile stress–strain measurements and thermogravimetric analysis. A SnO 2 content around 4wt.% gave a significant increase in membrane stiffness without losses in conductivity.