Abstract The paper describes the processes of water adsorption and desorption in a model hydrated acidic polymer, sulfonated and silylated poly-ether-ether-ketone (SOSiPEEK). Water uptake/loss experiments are carried out under conditions, where no membrane swelling occurs. The isotherms are described by analytical equations, inferred assuming that the surface of the membrane is uniformly covered by adsorbed molecules to an average depth of some monolayers. The adsorption depth is only controlled by diffusion of adsorbate from the surface towards the bulk; empty sites formed at the surface can accept further molecules of water. If the diffusion rate is fast enough, cumulative water uptake occurs. The water uptake kinetics is described considering the average penetration depth and the position of empty sites statistically formed at the surface of the membrane. The model also describes the desorption process, assumed to start at a prefixed time.