This work consists of the study of the extraction of solvent (acetone) from a polymeric (polyurethane) substrate during a leaching process. Total organic carbon (T.O.C.) is the main contaminant parameter in the leaching of these systems due to the solution of the acetone which is practically totally extracted from the polymeric system. During the leaching process, the water (polar) diffuses across the holes formed between the chains of polyurethane reacting at the same time with the isocyanate groups at the end of the chains, causing the reticulation or crosslinking of the polymer. On the basis of the experimental results, the amount of acetone diffused versus time in plane sheet systems was studied. A diffusion model based on the Fickian law was developed considering two stages. In the first stage, the water diffuses into the system across the polyurethane chains causing the reticulation of the polymer and dissolving the acetone. In the second stage, the diffusion of acetone in water, which occupies the holes or spaces between the reticulated polymer, takes place. The diffusion kinetics in these kinds of systems are similar to the diffusion kinetics in a rigid solid, considering an effective diffusivity of the acetone through the system.