The focus of this work is to analyze the influence of water cluster presence upon water vapour permeation through a polyester based polyurethane membrane. In this way modulated scanning calorimetry (MDSC) investigations have been done in order to characterize water states inside the film at various concentrations; Zimm and Lundberg's cluster integral has been adopted to have an analytic confirm of water molecule aggregate presence and the dimension of them. Transport analyses have been based upon solution diffusion model thus sorption and diffusion have been characterized independently. Specific mathematical models have been employed for such processes, as engaged species induced clustering (ENSIC) model as regards sorption, while free volume theory and modern free volume theory for diffusion. Particular attention has been paid upon diffusion coefficient calculation because of cluster presence; in this way, it was followed Crank's procedure opportunely modified. In the end has been compared moisture vapor transmission rate results with those ones obtained by means of a new mathematical transport model based upon recent equations that well describe sorption and diffusion when cluster formation occurs; it is also considered the influence of swelling on permeation. (C) 2003 Elsevier B.V. All rights reserved.