The distribution coefficients D-2/1 (Henderson-Kracek, Chlopin) of trace amounts of ions M2+ ( Ni2+, Mg2+, Cu2+, Co2+, Mn2+, Cd2+, Ca2+) with ZnSeO4 center dot 6H(2)O have been determined. The values of D-2/1 range from 2.93 (for Ni) to < 0.03 (for Ca), but most of them are nearly I and corresponding ions cannot be effectively removed from ZnSeO4. 6H(2)O during its crystallization. The dependences of coefficients on various chemical and crystal chemical factors have been analyzed. It has been found that ionic radii. parameter p as well as number of molecules of crystallizing water affect significantly D-2/1 coefficient. It has been proved possible to derive an empirical formula, which permits estimating D-2/1 coefficients at average relative error not exceeding 25%. (C) 2010 Elsevier B.V. Ali rights reserved.