Composite polymer membranes were prepared by incorporation of modified layered inorganic clay (Laponite) into Nafion membranes. The modification of Laponite particles was performed by the plasma activation process and the chemical grafting of sulfonic groups as well as by the direct plasma sulfonation. The modified Laponite particles incorporated into a polymer film contribute to an increase of the proton conductivity of composite membranes at high temperatures (85 degrees C). For the first time the water vapor sorption properties of the modified clay and the composite membranes were investigated varying water activity conditions (from 0.1 to 0.95) by means of microgravimetry. The BET adsorption model as well as models of Park and Feng were applied to the experimental data. The parameter values (concentration of the adsorbed water, specific capacity and different aggregation constants) were found to agree well with the measured ones. (C) 2010 Elsevier B.V. All rights reserved.