The present study provides an efficient process for the high-yield production of formic acid (24%) by reduction of carbon dioxide (CO2) with potassium borohydride at ambient conditions. The effects of reaction temperature, CO2 pressure and borohydride concentration have been investigated. For a 0.5 M borohydride solution, 0.15 mol/L of formic acid were produced at room temperature and ambient pressure with yields increasing at higher pressures. A time-resolved in situ H-1 and B-11 nuclear magnetic resonance (NMR) technique was firstly developed to monitor the elementary reaction processes under real working conditions. Direct evidence is given for the formation of H-2, HD and a hydroxyborohydride intermediate (BH3OH-) formed during borohydride decomposition indicating that the source of the hydrogen gas comes from both the borohydride anion and water, while borohydride works as a watersplitting reagent. Consequently, a reaction mechanism involved in both borohydride hydrolysis and CO2 reduction has been established. (C) 2015 Elsevier Ltd. All rights reserved.