The complex equilibria between Ba2+, H+ and thorin ions (L5-) were studied at 25 ºC in 70 % isopropanol and 0.1 M sodium (perchlorate) medium by means of potentiometric titration using a glass electrode. The total concentrations of thorin and Ba2+ were less than 2 x 10-4 M and 8 x 10-5 M, respectively. The pH range was 3 - 10. The data were evaluated with the generalised least square program LETAGROP. The results indicate that three proton compelxes are formed: HL4-, H2L3- and H3L2-, with the following step acidity constants: log K1 = 11.01, log K2 = 8.27 and kog K3 = 3.77. The main barium complexes are Ba(HL)2 6- and BaL2 8- with the stability constants log K222 = 15.70 and log K02 = 27.47