In this study, the effects of the addition of multi-wall carbon nanotubes (MWCNTs) into a microporous layer (MPL) containing Vulcan XC72R on the oxygen reduction reaction (ORR) were studied. We tested various percentages of MWCNTs and Vulcan XC72R in the MPLs of gas-diffusion electrodes (GDEs) with various Pt loadings in the catalyst layer. The performance of the ORR in the electrodes was studied with linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronoamperometry. The structures of the MPLs were investigated by using scanning electron microscopy (SEM), mercury porosimetry (MP), and gas permeability. In addition, the optimum polytetrafluoroethylene (PTFE) content of the MPL was determined. Our results indicate that the performances of the GDEs are optimal under the following conditions: (a) 60 wt% MWCNTs and 40 wt% Vulcan XC72R with a Pt loading of 0.115 mg/cm(2); (b) 80 wt% MWCNTs and 20 wt% Vulcan XC72R with a Pt loading of 0.5 mg/cm(2); and (c) 40 wt% MWCNTs and 60 wt% Vulcan XC72R with a Pt loading of 1 mg/cm(2). (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.