Platinum-gold nanoparticles supported on multi-wall carbon nanotubes (MWCNT) with various Pt:Au ratios are prepared by microemulsion method for use as electrocatalysts in the oxygen reduction reaction (ORR). The influence of the Pt:Au ratio on the electrocatalytic activity during ORR is investigated in detail. The synthesized nanomaterials are analyzed by different physicochemical techniques. The electrocatalytic activity for oxygen reduction is evaluated using a rotating disk electrode (RDE) system. We report that the activity and oxygen reduction mechanism depends strongly on the Pt:Au ratio; in the case of Pt:Au ratio higher than 0.60, oxygen is reduced to water by a four electron process pathway, while at 0.33 ratio the reaction follows a two-electron-transfer mechanism. (C) 2015 Elsevier Ltd. All rights reserved.