The mass transport mechanism of water into polyimide films has been analyzed and modeled on the basis of the relevant findings of an in situ FTIR spectroscopic analysis, performed previously, which (i) identified a molecular mechanism of diffusion based on two water species, i.e., H2O molecules interacting with the carbonyl groups of the polyimide and self-associated water, and (ii) evidenced the establishment of an instantaneous nonlinear equilibrium between these species. To model water transport, it has been assumed that concurrent diffusion occurs of two species, i.e., single water molecules and water dimers, which display different mobilities. A nonlinear instantaneous equilibrium between the local concentrations of these two species has been imposed. This equilibrium relationship has been derived on the basis of a two-layer BET (Brunauer, Emmett, and Teller) theory for the water sorption isotherm. The proposed approach is able to give a good qualitative and quantitative interpretation of both sorption equilibrium and of sorption/desorption kinetics data collected for each of the two water species identified by FTIR spectroscopy.