The kinetics of the formation of the thiylperoxyl radical RSOO. from 2-mercaptoethanol in oxygenated solution and its decay have been studied by pulse radiolysis by monitoring the evolution of the optical absorption of RSOO. at 560 nm. The rate of the decay of the 560-nm absorption appears to be independent of the oxygen concentration even under conditions where reaction 1 becomes rate-limiting, which indicates that a pathway for the removal of RS./RSOO. must exist in addition to reactions 1, 2, 3, and 7. This may be a tautomerism involving the thiyl radical which in the presence of oxygen already at relatively small concentrations (the effect starting to overwhelm the equilibrium 22/23 near 5 x 10(-5) mol dm(-3) O-2) leads to an irreversible bleeding-off of thiyl radicals. At higher oxygen concentrations, the rate of RS. (HOCH2CH2S.) removal on account of steps 22 and 26 remains small compared to that of step 1 followed by 3 and 7. It is hypothesized that the species HOCH2CH(OO.)HSH through elimination of HO2. forms the thiocarbonyl compound HOCH2CH=S which upon thiophilic addition of thiolate gives rise to the disulfide, a product which is not specific to the route through reactions 22 and 26 because it is also formed as a consequence of reaction 3 from sulfenic acid and thiolate : reaction 1/2, RS. + O-2 reversible arrow RSOO.; reaction 3, RSOO. + RSH --> RSO. + RSOH; reaction 7, RSOO. --> RS(O)O.; reaction 22/23, RS. reversible arrow HO-CH2-CH-SH; and reaction 26, HO-CH2-CH-SH + O-2 --> HO-CH2-CH(OO.)-SH. The room temperature rate constants of these reactions have been determined : k(1) = 2.2 +/- 0.1 x 10(9) dm(3) mol(-1) s(-1), k(2) = 6.2 +/- 0.3 x 10(5) s(-1), k(3) = 2 x 10(6) dm(3) mol(-1) s(-1), k(7) = 2 x 10(3) s(-1), k(22) approximate to 10(3) s(-1), and k(23) approximate to 3 x 10(4) s(-1). In this system, it has been possible to determine the stability constant K-1/2 = [RSOO.]/([RS.][O-2]) also from the dependence of the maximal absorbance at 560 nm on the oxygen concentration. The value thus obtained is 3.3 X 10(3) mol(-1) dm(3), in good agreement with the value determined from the kinetics, k(1)/k(2) = 3.5 +/- 0.4 X 10(3) mol(-1) dm(3).