[en] Co-crystallization coefficients of lanthanide ethylsulphates [Ln(H₂O)₉](C₂H₅SO₄)₃ were determined and compared with the solubility of these salts. In contrast to the common opinion denying any simple correlation between co-crystallization coefficients and solubilities it was established that a simple relationship does exist for ethylsulphates of light lanthanides. It was concluded that in the case of light lanthanides an almost complete compensation of activity coefficients in the RATNER equation is due to the similar structure of the hydrated cations [Ln(H₂O)₉]³⁺ in both the solid and the aqueous phase. For ethylsulphates of heavy lanthanides such a compensation was not observed, since in this case the cations in the aqueous phase are probably hydrated by 8 water molecules, while in the solid phase the coordination number is still 9. The solutisolubility of Pm, Pu, Am and Cm ethylsulphates was determined by the method ''from matrix''. (author)