The CoxNi1-x(SeO3)center dot 2H(2)O (x = 0, 0.4, 1) family of compounds has been hydrothermally synthesized under autogeneous pressure and characterized by elemental analysis, infrared and UV-vis spectroscopies and thermogravimetric and thermodiffractometric techniques. The crystal structure of Co0.4Ni0.6(SeO3)center dot 2H(2)O has been solved from single-crystal X-ray diffraction data. This phase is isostructural with the M(SeO3)center dot 2H(2)O (M = Co and Ni) minerals and crystallizes in the P2(1)/n space group, with a = 6.4681(7), b = 8.7816(7), c = 7.5668(7) angstrom, beta = 98.927(9)degrees and Z = 4. The crystal structure of this series of compounds consists of a three-dimensional framework formed by (SeO3)(2-) selenite oxoanions and edge-sharing M2O10 dimeric octahedra in which the metallic cations are coordinated by the oxygens belonging to both the selenite groups and water molecules. The diffuse reflectance spectra show the essential characteristics of Co(H) and Ni(H) cations in slightly distorted octahedral environments. The calculated values of the Dq and Racah (B and C) parameters are those habitually found for the 3d(7) and 3d(8) cations in octahedral coordination. The magnetic measurements indicate the existence of antiferromagnetic interactions in all the compounds. The magnetic exchange pathways involve the metal orbitals from edge-sharing dimeric octahedra and the (SeO3)2- anions which are linked to the M2O10 polyhedra in three dimensions. (c) 2005 Elsevier Ltd. All fights reserved.