The fuel efficiency of borohydride electro-oxidation on carbon-supported Au and Ag electrodes was found to be highly dependent on the concentration ratio of [OH-]/[BH4] in the solution Near-8e reactions occurred when [OH-]/[BH4-] >= 5. However when [OH-]/[BH4] was smaller than 5, hydrogen gas was evolved and the fuel efficiency was reduced. Only 3e reaction stoichiometry was obtained at [OH-]/[BH4-] = 1. Detailed cyclic voltammetry (CV) studies revealed that both the Au/C and Ag/C showed different anodic waves in varied NaOH-NaBH4 solutions The CV analysis results suggest that BH3OH-, an intermediate possibly produced by homogeneous hydrolysis, is responsible for the electrochemical reaction at [OH-]/[BH4-] = 1 Comparison of CV voltammograms in borohydride solutions with that in the H-2-bubbled NaOH solution suggests that borohydride electro-oxidation on Au or Ag electrode is through direct BH4 oxidation rather than through a hydrogen ionization mechanism. it is concluded that there exists an inherent competition between two oxidizing species of OH and H2O during borohydride electro-oxidation, that is, if accessible OH ions are riot sufficient for each BH4- to accomplish the 8e electro-oxidation, part of BH4 will react simultaneously with H2O to generate hydrogen (C) 2009 Professor T Nejat Veziroglu Published by Elsevier Ltd. All rights reserved.