The structures of the hypophosphites KH(2)PO(2) (potassium hypophosphite), RbH(2)PO(2) (rubidium hypophosphite) and CsH(2)PO(2) (caesium hypophosphite) have been determined by single-crystal X-ray diffraction. The structures consist of layers of alkali cations and hypophosphite anions, with the latter bridging four cations within the same layer. The Rb and Cs hypophosphites are isomorphous.