MWNTs can be conveniently dispersed in Nafion solution on the basis of the special interactions between the sidewall of MWNTs and the hydrophobic domains of Nafion. Casting of the resulting mixture on electrode surfaces produced uniform composite films having wide electroanalytical applications. In this work, the electrochemical properties of the MWNTs-Nafion composite film on a glassy carbon electrode were systematically investigated by various electrochemical methods using incorporated europium(III) ions (Eu3+) as the probe. The voltammetric studies showed that the increase of MWNTs concentration in the composite film could effectively improve the redox currents of Eu3+ and reduce the peak separation, whereas the increase of Nation concentration generally increased both the redox currents and the peak separation. These results suggested the different roles of MWNTs and Nafion in the composite films. The electrochemical impedance spectroscopic (EIS) investigations showed that MWNTs mainly contributed to the charge transfer and mass transfer processes of the composite film through the increases of the electrode/electrolyte interfacial area and the film porosity while Nafion generally dominated the mass transport from the solution into the film via ion exchange. The potential application of the sensitive response of Eu3+ at the MWNTs-Nafion composite film in electroanalytical chemistry was evaluated. In the range of 0.04-100 mu M, the concentration of Eu3+ showed excellent linear relationships with the differential pulse voltammetric response with a low detection limit of 10 nM (S/N = 3) for 60 s accumulation at -0. 1 V. (c) 2005 Elsevier Ltd. All rights reserved.