In the presence of cysteine, osmium(VIII) in hydrochloric acid exhibits a well-defined maximum wave at ¹0:85 V (vs. SCE). Effects of the osmium's valence states and the coexisting anions on the reduction wave have been studied polarographically in acidic solution. The high oxidation states (e.g., osmium(VIII) and (VI)) in comparison with low oxidation states have an influence on the height of the hydrogen wave.Keywords: Osmium, Cysteine, Maximum wave, PolarographySince Herles and Vancura's [1] first observation in 1930 of an unusual wave during the polarographic analysis of blood serum solutions in ammonia buffer, the study of a catalytic wave arising from the electrode reaction that contains sulfur compounds has been the objective of numerous studies. In our previous article [2], we describe the fact that osmium(VIII) tetraoxide, in the presence of cysteine in hydrochloric acid, also results in an unusual polarographic wave. The article showed the following results for the electrode reaction:• in acidic solution, the osmium-cysteine yields a maximum wave; • the peak current reaches a maximum when the concentration ratio of osmium/cysteine is about 2000;• the peak current is enhanced by adding cysteine more than 1000 times compared to the case where no cysteine exists;• in the presence of disulfide compounds, the wave does not occur;• the adsorption at the peak potential is observed from the result of the electrocapillary curves.However, several important questions relevant for the chemical reaction in bulk solution and the electrode reaction on the electrode surface have remained unanswered. Different experimental conditions were used because this osmium-cysteine system is different from other studies. Many studies of the 'Bridicka's current' had been carried out in the pH range of 4 to 9 and metal ions such as Co 2þ or Ni 2þ were employed. Prior studies using platinum metal also used low valence state ions in mixed solvent [3]. In our study, however, a high valence state of osmium ions was used in acidic solution. It is supposed that the chemical, nonelectrochemical, redox reaction which turns the valence state into the low state of osmium takes place in the bulk because osmium tetraoxide and cysteine are oxidizing and reducing agents, respectively. Therefore, it is unknown whether the high or low states of osmium are concerned with the appearance of a maximum wave. Kadlecek et al.[4] studied the catalytic activity of structural similar thiols and found that the activity decreases with the stability decrement of their complexes with Co II ions. Few studies [5] associated with complexing between platinum metals and low molecular weight thiols have been carried out. With respect to osmium, Medyantseva et al. [3] determined Os III with 8-mercaptoquinoline in a mixture of dimethylformamide with an acetate buffer. Aliphatic thiol compounds such as cysteine, however, have not been investigated. This communication examines the interaction between osmium and cysteine, and the electrochemical behavior of the sys...