The in situ ion exchange method has been employed to prepare carbon nanotubes (CNT) supported Pt electrode, in which CNT is functionalized with two-step oxidation, namely electrochemical oxidation and chemical oxidation. X-ray photoelectron spectroscopy (XPS) confirms that two-step oxidation produces more carboxylic acid groups. Transmission electron microscopy (TEM) shows that Pt nanoparticles are highly dispersed on the CNT surface. Electrochemical measurements show that the resultant Pt/CNT electrode treated by two-step oxidation exhibits the largest electrochemical surface area and the highest activity for oxygen reduction reaction (ORR) among the investigated electrodes. This can be attributed to the fact that the two-step oxidation treatment produces more carboxylic acid groups which is the determining factor for Pt loading and dispersion via ion-exchange. (C) 2011 Elsevier B.V. All rights reserved.