The hydrolysis of the borohydride ion according to the equation (i) is catalysed by nickel–boron and cobalt–boron, BH4–+ 3H2O → H2BO3–+ 4H2(i) alloys. The metal-catalysed hydrolysis has been studied at 22·0 °C by measurement of the hydrogen gas evolved. Analysis of the gas evolved when the reaction is carried out in deuterium oxide shows that approximately half the hydrogen evolved originates from the borohydride and half from the solvent. The deuterium content of the gas increases with reaction time. In mixtures of H2O and D2O, preferential hydrogen evolution occurs and isotope separation factors of 8·5 (for a nickel–boron catalyst) and 9·2 (for a cobalt–boron catalyst) have been derived.