A detailed mathematical model for the direct borohydride/O-2 fuel cell is developed. The activation polarizations, mass-transport limitations and resistances to charge transport in each component of the cell are explicitly incorporated. The anode kinetic mechanism is based on direct borohydride oxidation, borohydride hydrolysis and the full Tafel-Volmer-Heyrovsky mechanism for hydrogen evolution and oxidation. The mixed potential at the anode is calculated using the mixed-potential theory. The model results are compared to experimental data across a range of operating conditions and component properties, including the reactant concentrations, the anolyte flow rate, the ionomer volume fractions and the membrane/ionomer conductivity. A good qualitative fit to the experimental data is demonstrated. In order to gain insight into the anode reaction mechanism, the performance on both Pt and Ni anodes is simulated and compared to experimental observations. A detailed analysis of the results provides an explanation for the different performance on these catalysts. (C) 2012 Elsevier B.V. All rights reserved.