A method is described for the preparation of di- and tetra- sodium hypophosphate and for the conversion of these salts, by an ion exchange technique, into pure hypophosphoric acid. Methods are also described for the preparation of various metal hypophosphates, including a stable, insoluble, ferrous hypophosphate which has not been reported previously. Solutions of iron (II), iron (III) and cobalt (II) in aqueous solutions of hypophosphoric acid have been studied by cation and anion exchange batch techniques, and the existence has been demonstrated of soluble ferrous and ferric hypophosphate complexes. In addition, the spectra of these systems have been studied where possible. From the results obtained from these experiments, the metal: ligand ratio and the charge of the complexes have been deduced. Further ion exchange batch equilibrium experiments have been carried out on the iron (II) and iron (III) hypophosphate systems, using tracer quantities of metal and very carefully controlled conditions of pH and ionic strength and, from the results, values for the stability constants of the complexes have been calculated.