In this study, diffusion coefficients of cations (Cu(II), Ni(II), and Fe(III)) through a Nafion membrane were measured with a two-compartment cell and using a technique that monitored pH change during ion exchange. In the cell experiments the single ion diffusion coefficients of Cu(II), Ni(II), and Fe(III) were 1.04 x 10(-6), 8.36 x 10(-7), and 4.24 x 10(-7) cm(2)/s, respectively. In the pH monitoring experiments they were similar (1.26 x 10(-6) and 6.23 x 10(-7) cm(2)/s for Cu(II) and Ni(II), respectively). As expected, the trivalent cations diffused slower than the divalent cations through the membrane. Diffusion coefficients increased with decreasing pH especially when pH was below 1. The Arrhenius equation well characterized the variations in the diffusion coefficients through the Nafion membrane measured at different temperatures. In the presence of other ions the measured diffusion coefficients increased slightly. In the ceramic membrane diffusion coefficients were larger than in the Nafion membrane (3.64 x 10(-6), 3.04 x 10(-6), and 1.68 x 10(-6) cm(2)/s for Cu(II), Ni(II), and Fe(III), respectively). The approximate tortuosity factors of the Nafion membrane and ceramic membrane were 5 and 1.6 for Cu(II), respectively. The species concentration variation in the system was accurately predicted using a model based on Fick's second law. (C) 2003 Elsevier B.V. All rights reserved.