The solubilization behavior of acetylacetonate complexes of lanthanoids(III) (Nd, Sm, Tb, and Tm) in sodium dodecyl sulfate micelles has been quantitatively evaluated by means of micellar capillary electrophoresis. From analysis of the migration times of the lanthanoid complexes, assuming incorporation of the cationic bis-acetylacetonates as well as the neutral tris-acetylacetonates into the anionic micelles, the distribution constant of each species between the micelles and water (K-mc/w) was determined. The distribution constants obtained were compared with those of the trisacetylacetonates between heptane and water (K-hep/w) The K-mc/w value of the neutral complex is smaller than that of the cationic one but much larger than the K-hep/w value. The K-mc/w and K-hep/w values of the neutral complex increase with increasing atomic number of the lanthanoid, whereas the opposite tendency was observed for K-mc/w of the cationic complex.