Stabilization of dissolved trivalent thallium, Tl(III), in water by complexation with humic acid and its analogue (poly(acrylic acid), PAA) was studied. Chromatographic analysis showed that Tl(III) was stabilized by the complexation, which allowed us to estimate the stability constant of Tl(III)-humate complex. Cyclic voltammograms of Tl showed that the redox potential of Tl(III)/Tl(I) redox couple in the presence of PAA was lower than that without PAA. These results suggest that Tl(III) can be present in natural water by the stabilization of Tl(III) rather than Tl(I) due to the formation of humate complex.