This study concerns the synthesis and fabrication of manganese oxide nanostructures via electrodeposition onto Pt electrodes from aqueous solution of 0.1 M Na2SO4 containing 0.1 M Mn(CH3COO)(2). The structural and morphological characterization of the electrodeposited manganese oxide was performed with scanning electron microscopy and high-resolution transmission electron microscopy. These techniques revealed the formation of intersected nanorods of single-crystalline gamma-MnOOH (manganite phase), in agreement with the results of the X-ray diffraction data. Furthermore, a significant enhancement of the electrocatalytic activity of Pt toward the oxygen reduction reaction (ORR) has been observed in O-2-saturated 0.1 M KOH upon the electrodeposition of gamma-MnOOH nanorods. A surface coverage of MnOOH (theta) of ca. 0.3 caused a positive shift of the peak potential of the ORR by about 80 mV compared to that obtained at bare Pt electrode. Similar Tafel slopes (approximate to-60 mV/decade) were obtained at the bare and the modified Pt electrodes, indicating that the ORR proceeds via the same rate-determining step at the aforementioned electrodes. (c) 2007 The Electrochemical Society.