[en] Complex formation equilibria in lanthanoid (III)-3-bromo-5-sulfosalicylate (H₂L^-) system has been studied by potentiometric titration over the pH range 2-9. The measurements were performed in 0.10 M NaClO₄ at 25 deg. C. The data indicate the formation of the species [Ln(HL)]⁺, [LnL], [LnL₂]^3-, [Ln(OH)L]^- and [Ln(OH)₂L]^2-. Enthalpy values were obtained for the [Ln(HL)]⁺ complexes by calorimetric titrations. The mode of complexation was studied by ¹³C NMR spectroscopy. The results are compared with data for complexation by various aromatic hydroxycarboxylic acids. (au) (22 refs.)