The interactions of Sc3+ and Y3+ ions with disodium 1,2-dihydroxybenzene-3,5-disulfonate (Na2H2L, where H2L2- = Tiron) were investigated in aqueous solution by means of potentiometric and spectroscopic methods. The coordination of Tiron to Sc3+ and Y3+ takes place through two phenolic oxygen atoms of catecholate ion in different stoichiometries. Thus, the binding of Tiron to Sc3+ occurs either in 1 : 1 or 1 : 2 molar ratios; they have resulted by the formation of [ScL](-) and [ScL2](5-) type complexes, respectively. On the other hand, Y3+ ion behaves like Th4+ and Ln(3+) ions toward Tiron. It forms [YL](-) type complex in 1 : 1 molar ratio; but in 1 : 2 or in higher molar ratios, only unique [Y2L3](6-) type complex formation takes place. The formation constants of [ScL](-), [ScL2](5-), [YL](-), and [Y2L3](6-) complexes were determined by analysis of the potentiometric data in ionic medium of 0.1 M KNO3 or NaClO4 at 25degreesC. The hydrolytic reactions of Sc(III) and Y(III) complexes with Tiron were determined from potentiometric data and the formation constants of [ScL(OH)](2-) and [YL(OH)](2-) were also calculated.