The hydrolysis of the hypophosphite ion occurs according to equation (i) and is catalysed by nickel–phosphorus, H2PO2–+ H2O [graphic omitted] → H2PO3–+ H2(i) cobalt–phosphorus, and nickel–boron alloys. The stoicheiometry and the kinetic behaviour have been examined by measuring the hydrogen evolved in the catalysed hydrolysis at 98·7 °C. Further information concerning the reaction mechanism has been obtained by carrying out the reaction in D2O or D2O–H2O mixtures. The reaction can be followed by 1H n.m.r. measurements of the concentrations of the species H2PO2– and H2PO3–. The deuterium content of the gas evolved has also been studied. A reaction mechanism is proposed involving simultaneous reactions in which the hypophosphite molecule exchanges either one or both phosphorus-bound hydrogen atoms with the solvent.