The complexes formed in the dimethylthallium(III) (Me2Tl+), glutathionate (EGC3−) and hydrogen ion system in aqueous solution at 37 °C and I = 150 mmol dm−3 (NaCl) have been characterised by means of glass-electrode potentiometry. Glutathione protonation constants were found to be 9.123 ± 0.007, 17.42 ± 0.01, 20.78 ± 0.02, and 22.93 ± 0.02. Formation constants for the complexes [(Me2Tl)EGCH]− and [(·Me2Tl) EGC]2− were found to be 11.19 ± 0.03 and 2.39 ± 0.02, respectively. Particular attention has been paid to the evaluation of the effect of possible systematic errors on the constant values determined. Reliable standard deviation estimations have been made by applying a Monte Carlo calculation technique.