The single molybdenum oxide (MoO2) crystals down to 5 nm in diameter on carbon (denoted as C-MoO2) are synthesized based on ion-exchange principle for the first time. The structures, morphologies, chemical and electrocatalytic performances of as-synthesized nanomaterials are characterized by physical, chemical and electrochemical methods. The results indicate that electrocatalysts made with Pt nanoparticles supporting on C-MoO2 (denoted as Pt/C-MoO2) are highly active and stable for oxygen reduction reaction (ORR) in fuel cells. A mass activity of 187.4 mA mg(pt)(-1) at 0.9 V is obtained for ORR, which is much higher than that on commercial Pt/C (TKK) electrocatalyst (98.4 mA mg(pt)(-1)). Furthermore, the electrochemical stability of Pt/C-MoO2 is more excellent than that of Pt/C (TKK). The origin of the improvement in catalytic activity can be attributed to the synergistic or promotion effect of MoO2 on Pt. The improvement in electrochemical stability is due to the strong interaction force between Pt and MoO2. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.