Gas diffusion electrodes (GDE) composed of: Toray carbon paper gas diffusion layer (unteflonated and teflonated with 30%wt PTFE, respectively) and MnO2 supported on Vulcan XC72R catalyst layer with 20%wt PTFE, were investigated in 6 M KOH without and with O-2, respectively. Four sources of MnO2 (Sigma-Aldrich, Tronox, Riedel and Merck Inc.) were comparatively studied by electrochemical methods accompanied by XRD characterization. Cyclic voltammetry scans of GDE in N-2-purged electrolyte were used to estimate the Tafel slopes for Mn(IV) reduction and to identify active Mn(IV) sites that play an important catalytic role. Two oxygen reduction reaction (ORR) mechanisms were identified by porous rotating disk electrode (PRDE) polarization as a function of electrode potential. At high potentials (above -300 mV vs. MOE) the O-2/HO2- step is mainly catalyzed by the quasi-unreduced MnO2 surface and active sites on the carbon support, while at potentials more negative than -300 mV, the redox catalysis by Mn(III) prevails. The main catalytic sites for the second step HO2-/OH-, are Mn(III) sites. The hydrophobic property of the porous electrode is important due to the effect on peroxide desorption/readsorption on the electrode surface. PRDE and flow cell experiments revealed the Sigma-Aldrich gamma-MnO2 was the most active for ORR. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.061202jes] All rights reserved.