This chapter discusses some examples of (1) the reactions of thiols with reactive oxygen species and free radicals, (2) the nucleophilic addition of thiolate to some electrophiles, and (3) thiol–disulfide exchange reactions, in particular those involved in the formation and cleavage of protein disulfide bonds. The examples permit the discussion of various methods and techniques employed for the kinetic measurements, exhibiting a broad range of time resolution, and the concepts may easily be applied to any kinetic problem of interest. For many kinetic measurements, it is sufficient to follow the reaction of a thiol by monitoring the final formation of a stable disulfide, which can be conveniently done by means of high-performance liquid chromatography (HPLC). However, in some instances it is necessary to describe the one-electron oxidation of a thiol to its corresponding thiyl radical accurately. Such measurements require fast techniques—such as pulse radiolysis or laser-flash photolysis—owing to the short half-life for the formation and subsequent reactions of thiyl free radicals.