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Here, we report the results of kinetic and mechanistic studies of the first step of reaction between thiols (viz., cysteine, N-acetylcysteine, mercaptoethanol, thioglycolic acid and 5-thio-2-nitrobenzoic acid (TNB)) and selenite producing thiol-S-Selenites (RS-SeO2−) in aqueous solutions of various acidity. In this set of aliphatic thiols, their reactivity toward selenite in neutral medium is directly dependent on the acidity of thiol group, whereas the stability of RS-SeO2− is affected by the interaction with groups adjacent to S–Se bond. In the case of TNB, the reaction with selenite in neutral medium proceeds much slower than processes involving aliphatic thiols due to low nucleophilicity of TNB thiol group. Using Ellman's reagent, we demonstrated that selenite binds substantially weaker to thiol group of cysteine residue in bovine serum albumin than to thiol group of free cysteine. [ABSTRACT FROM AUTHOR]