Compounds of general formula (Mg,M)(3)(AsO4)(2) . 8H(2)O (M = Ni2+), Co2+ have been synthesized and characterized, The crystal structure of the (Mg2.5Ni0.5)(AsO4)(2) . 8H(2)O compound has been refined using the Rietveld method. The final reliability factors are R(p) = 5.62, R(wp) = 7.06, and R(B) = 5.29. Crystallographic studies reveal that both the ionic radius of the cations and the symmetry of the octahedral sites can play an important role in determining the cation ordering in these compounds. Diffuse reflectance spectra indicate a significant covalence in the nickel and cobalt-oxygen bonds. The analysis of the magnetic measurements and the structural features shows a strong dimer ferromagnetic interaction in the compounds.