The complex formation equilibria of lanthanum(III) with 1,2-dihydroxy-3,5-di-sulphonate (Tiron, H2L2-,), and 4-nitro-catechol (4-NCAT, H2L)] were investigated potentiometrically at 25.0 +/- 0.1 degrees C and in ionic strength of 0.2M KCl. The protonation constants for these catechol derivatives and the formation constants of their complexes formed with La(III) were determined. These catechol derivatives are coordinated by phenolic hydroxyl groups to La(III), so that Tiron forms only 1:1 LaL- type complex in acidic region, regardless of the amount of excess Tiron present; but in neutral and basic region the occurrence of LaL(OH)(2-) type hydroxo complex was noticed. 4-NCAT forms both LaL+ and LaL2-type complexes. The stabilities of 1:1 La(III) complexes of these catechol derivatives decrease in the order: Tiron>4-NCAT. As a result of increase in ionic radius, the stabilities of complexes formed between group III elements and these substituted catecholates decrease in the following order: Sc(III)>Y(IIl)>La(III).