A thin Nafion (R) ion exchange membrane was coated on a graphite electrode, and then impregnated by a metal complex couple of Os(bpy)(3)(2+/3+), which served as a typical system to investigate the water content change induced by ion exchange. Cyclic voltammetry and FTIR reflection-adsorption spectroscopic methods were employed to characterize the complex loading, electrochemical behavior of the impregnates inside the film and the water content change. A direct observation of the water content reduction induced by the cation impregnation in the Nafion (R) film was achieved. A monotonic relationship between the water reduction percentage and the Os(bpy)(3)(2+/3+) loading was also obtained, that is, the heavier the complex loading, the more the water content will be reduced. The FTIR spectroscopic results suggested that the structure of the Nafion (R) film could also be changed with water content reduction. (c) 2005 Elsevier Ltd. All rights reserved.