The diffusion of the nitrogen heterocyclic base, 1,10-phenanthroline, into the perfluorosulfonated ion-exchange polymer membrane, Nafion(R) 117, has been studied in aqueous solutions via specially designed diffusion cells. Solutions of identical composition were mechanically stirred and the concentration of the neutral species was monitored spectrophotometrically with time. A mathematical model describing the system was developed. It takes into account the coupled character of the diffusion and the protolytic equilibria within the Nafion(R) membrane. The model was numerically solved using an implicit alternating-direction finite difference method and the corresponding software was written in ANSI C. Simulated extraction curves were fitted to the experimental data, in which the diffusion coefficient of 1,10-phenanthroline in the Nafion(R) membranes was determined and the relevant mechanisms of extraction discussed.