The first three stepwise formation constants of the 2-hydroxy- 3- methylbutanoate (isopropylglycolate), 2, 3-dimethyl- 2-hydroxybutanoate (methylisopropylglycolate), 3, 3-dimethyl-2-hydroxybutanoate (tertiarybutylglycolate), and 2-hydroxy-2, 3, 3-trimethylbutanoate (methyltertiarybutylglycolate) complexes of the trivalent rare earths and yttrium were determined at 25° C and an ionic strength of 0. l molar (NaCl0₄ ). The stabilities of the complexes followed the ionic radius trend quite regularly. The stabilities generally decreased with increasing complexity of the alkyl substituents on the ligand, but replacing a hydrogen atom by a methyl group invariably enhanced the stabilities of the complexes, especially in the case of the heavier, smaller rare-earth ions. It was postulated that the stabilities of the heavier rare-earth complexes with the methyl-substituted homologues were enhanced because of a smaller, more favorable hydroxyl-carboxyl bond angle.