Modifying pristine graphite felt (GF) active electrode, highly active platinum/multiwalled carbon nanotubes (Pt/MWNTs) electrocatalyst, helps in developing new V(III)/V(V) vanadium redox flow battery (VRFB) and the improving the performance of the V(II)/V(V) battery. Cyclic voltammetry analyzes demonstrated that Pt/MWNT electrocatalyst has an excellent capacity for enhancing V3+/VO2+ and VO2+/VO2+ redox reactions, and both reactions are quasireversible. The V3+/VO2+ redox couple provides the V(III)/V(V) battery redox reaction. The VO2+/VO2+ and V3+/VO2+ redox reactions occurred in the pristine GF of the positive active electrode and in the Pt/MWNT + GF of the negative active electrode, respectively, using the same electrolyte (1 M VO2+ -1 M H2SO4). At the operating current densities of 12 and 20 mA cm (2), the cell coulombic efficiency reached 93.75% and 83.88%, respectively. A test V(II)/V(V) battery was assembled with the positive active electrode of Pt/MWNT + GF and the negative active electrode of the GF. The charge-discharge curves showed a slight IR drop in the active electrode, which was likely caused by the Pt/MWNT electrocatalyst. At an operating current density of 20 mA cm(-2), the cell energy efficiency was 11.58% higher than that without an electrocatalyst. The results show the potential of Pt/MWNT catalysts in VRFB applications. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.003210jes] All rights reserved.