The interactions of scandium(III) and yttrium(III) ions with catechol derivatives; catechol($H_2L$, CAT), 4-nitrocatechol($H_2L$, 4NCAT) and 2,3-dihydroxynaphthalene- 6-sulphonic acid($H_2L$, DHN) were investigated in different metal ion:ligand mole ratios, by potentiometry and spectrophotometry.The stoichiometries of formed complexes have been defined by Job's method and formation curves have been drawn for some complexes, whenever possible. The stability contants of the fourteen formed complexes in definite stoichiometries have been determined, in an ionic medium of 0.1M$KNO_3$ or $NaClO_4$ at a temperature of 25°C. Noticeably, complexes of scandium(III) ion formed at much lower pH values than complexes of yttrium(III) ion, and its catechol complexes were more stable than complexes formed with yttrium(III) ion. The dependence on ionic strength of stability constants of Sc(III) and Y(III) were defined by comparing them with values from the literature whenever possible. The stabilities of complexes of Sc(III) or Y(III) with catechol derivatives in (1:1), (1:2) and (1:3) mole ratios were compared by considering their possible structures.