The title compounds, hexaaquacobalt(II) bis(hypophosphite), [Co(H(2)O)(6)](H(2)PO(2))(2), and hexaaquacobalt(II)/nickel(II) bis(hypophosphite), [Co(0.5)Ni(0.5)(H(2)O)(6)](H(2)PO(2))(2), are shown to adopt the same structure as hexaaquamagnesium(II) bis(hypophosphite). The packing of the Co(Ni) and P atoms is the same as in the structure of CaF(2). The Co(II)(Ni(II)) atoms have a pseudo-face-centred cubic cell, with a = b approximately 10.3 A, and the P atoms occupy the tetrahedral cavities. The central metal cation has a slightly distorted octahedral coordination sphere. The geometry of the hypophosphite anion in the structure is very close to ideal, with point symmetry mm2. Each O atom of the hypophosphite anion is hydrogen bonded to three water molecules from different cation complexes, and each H atom of the hypophosphite anion is surrounded by three water molecules from further different cation complexes.