AbstractMixed crystals (MgxNi1–x)SeO4· 6 H2O and (MgxCu1–x)SeO4· 5 H2O have been prepared studying the solubility in the MgSeO4–NiSeO4–H2O and MgSeO4–CuSeO4–H2Osystems at 25 °C. It has been shown that the monoclinic structure of MgSeO4· 6 H2O is unstable and undergoes a change into tetragonal structure due to the included nickel ions (about 4 at %). The lattice parameters of (MgxNi 1–x)SeO46 H2O have been calculated. It has been established that the magnesium ions incorporate isodimorphously in the crystal structure of CuSeO4· 5 H2O which could be an indication of the existence of MgSeO4· 5 H2O isostructural with the triclinic CuSeO45 H2O. The distribution coefficients of the salt components between the liquid and solid phases h