Tetra-aqua-(18-crown-6)cerium(III) hexa-cyano-ferrate(III) di-hydrate, [Ce(C12H24O6)(H2O)4][Fe(CN)6]·2H2O, and tetra-aqua-(18-crown-6) neodymium(III) hexa-cyano-ferrate(III) dihydrate, [Nd(C12H24O6)(H2O)4][Fe(CN)6] ·2H2O, are iso-morphous and isostructural in the C2/c space group, where the cations, which contain ten-coordinate lanthanoid centres, lie across twofold rotation axes and the anions lie across inversion centres. In these compounds, an extensive series of O - H⋯O and O - H⋯N hydrogen bonds links the components into a continuous three-dimensional framework. Triaqua-(18-crown-6)lanthanoid(III) hexa-cyano-ferrate(III) di-hydrate, [Ln(C12H24O6)(H2O)3][Fe(CN)6]·2H2O, where Ln = Sm, Eu, Gd or Tb, are all isomorphous and isostructural in the P space group, as are triaqua-(18-crown-6) gadolinium(III) hexa-cyano-chromate(III) dihydrate, [Gd(C12H24O6)(H2O)3][Cr(CN) 6]·2H2O, and triaqua-(18-crown-6)gadolinium(III) hexa-cyano- cobaltate(III) dihydrate, [Gd(C12H24O6)(H2O)3][Co(CN)6]·2H2O. In these compounds, there are two independent anions, both lying across inversion centres, and the lanthanoid centres exhibit nine-coordination; in the crystal structures, an extensive series of hydrogen bonds links the components into a three-dimensional framework. © International Union of Crystallography 2007.