The separation of a samarium (90%) and europium (10%) mixture in ol-hydroxy isobutyric acid was performed in a new countercurrent electromigration system. The mobilities of these elements were estimated, and samarium of better than 99.9% purity was obtained. The equilibrium of multicoordinate complexes of these elements with alpha-hydroxy isobutyric acid (alpha-HIBA) plays an important role in the separation process. The equilibrium concentrations of the involved species were calculated by a computational procedure, and a kinetic study of the complexation reaction was also performed.