Polymer membranes with in situ formed silver nanoparticles were prepared by dissolving together AgBF4 and poly(amide 12)-poly(tetramethyleneoxide) block copolymer in an organic medium then evaporating the solvent medium. UV-vis spectroscopic studies evidenced the formation in the solution of silver nanoparticles right whose content and size increase with the exposure time under the irradiation of the spectrophotometer lamp. The solid hybrid film changed its color with time under ambient conditions, indicating an evolution of the inorganic matters in the polymer matrix. The transmission electron microscopy showed nanoparticles whose size increases from 18 to 27 nm in the stabilized form in the polymer. The water vapor uptake in the membranes with different AgBF4 contents studied by microgravimetry allowed us to determine the sorption isotherms at 25 degrees C. The sigmoidal BET-II type isotherms were fairly well fitted with Park's equation, where the concave part was fitted with a Langmuir and a Henry sorption term, while the convex part was fitted with an exponential term in Park's equation that takes into account the clustering of water molecules in the polymer matrix. We suggest that at low water activity, water molecules adsorb on Langmuir sites-probably on the nanoparticle surface-and at high activities water molecules form aggregates in the polymer matrix. (c) 2007 Elsevier Ltd. All rights reserved.