The mechanism of the (OH)-O-.-induced oxidation of S-ethylthioacetate (SETAc) and S-ethylthioacetone (SETA) was investigated in aqueous solution using pulse radiolysis and steady-state gamma radiolysis combined with chromatographic and ESR techniques. For each compound, (OH)-O-. radicals were added, as an initial step, to the sulfur moiety, forming hydroxysulfuranyl radicals. Their subsequent decomposition strongly depended on the availability of alpha- or beta-positioned acetyl groups, pH, and the thioether concentration. For SETAc, which contains the a-positioned acetyl group, hydroxysulfuranyl radicals SETAc(>S-.-OH) subsequently decay into secondary products, which do not include intermolecularly three-electron-bonded dimeric radical cations, even at high concentrations of SETAc. At low pH, these observations are rationalized in terms of the highly unstable nature of sulfur monomeric radical cations SETAc(>S+.) because of their rapid conversion via deprotonation to the alpha-(alkylthio)alkyl radicals H3C-(CH)-C-.-S-C(=O)-CH3 (lambda(max) = 420 nm). However, at low proton concentrations, the alpha-positioned acetyl group destabilizes SETAc(>S-.-OH) radicals within a five-membered structure that leads to the formation of alkyl-substituted radicals, H3C-CH2-S-C(=O)-(CH2)-C-.. A somewhat different picture is observed for SETA, which contains a beta-positioned acetyl group. The main pathway involves the formation of hydroxysulfuranyl radicals SETA(>S-.-OH) and alpha-(alkylthio)alkyl radicals H3C-CH2-S-(CH)-C-.-C(=O)-CH3 (lambda(max) = 380 nm). The latter radicals are highly stabilized through the combined effect of both substituents in terms of the captodative effect. At low pH, SETA(>S-.-OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations SETA-(>SthereforeS<)(+) (lambda(max) = 500 nm), especially for high SETA concentrations. In contrast, at low proton concentrations, SETA(>S-.-OH) radicals decompose via the elimination of water, formed through intramolecular hydrogen transfer within a six-membered structure that leads to the formation of alkyl-substituted radicals, H3C-CH2-S-CH2-C(=O)-(CH2)-C-.. The latter radicals undergo a 1,3-hydrogen shift and intramolecular hydrogen abstraction within the six-membered structure, leading to the alpha-(alkylthio)alkyl radicals H3C-CH2-S-(CH)-C-.-C(=O)-CH3 and H3C-(CH)-C-.-S-CH2-C(=O)-CH3, respectively. To support our conclusions, quantum mechanical calculations were performed using density functional theory (DFT-B3LYP) and second-order Moller-Plesset perturbation theory (MP2) to calculate the bond-formation energies of some key transients and the location and strength of their associated optical absorptions.