Formation constants for thallium(I) complexes of L-cysteine (CysH2), DL-penicillamine (PenH2), N-acetyl-L-cysteine (AcyH2), and N-acetyl-DL-penicillamine (ApeH2) in aqueous solution have been determined in 150 mmol dm−3 NaCl medium at 37°C by potentiometric titrations using a glass electrode. Glycine has been used as a model for simple amino acids. The experimental data may be explained by the formation of the complexes Tl(Cys)−, Tl(Cys)H, Tl(Pen)−, Tl(Pen)H, Tl(Acy)−, and Tl(Ape)− with log formation constants 3.26, 11.28, 3.60, 12.05, 2.27, and 2.45, respectively. Analysis of the results obtained and comparison of thallium(I) complexing ability with that of dimethylthallium(III) seem to indicate that thallium(I) toxicity does not directly stem from its interference with the metabolism of sulphur-containing compounds.