One-electron oxidation of acetyl thiolate (CH3C(O)S-) was achieved by hydroxyl (OH .) and azide (N-3.) radicals in aqueous solution. The resulting acetylthiyl radical (CH3C(O)S .) absorbs in the wavelength region 300-550 nm, with a maximum extinction coefficient of 3900 M-1 cm(-1) at 440 nm. With N-3./N-3(-) as a reference couple, the reduction potential E degrees(CH3C(O)S ./CH3C(O)S-) was measured to be 1.22 V vs NHE. Using a pK(2) of 3.35 for thioacetic acid (CH3C(O)SH), the standard reduction potential E degrees(CH3C(O)S ., H+/CH3C(O)SH) is calculated to be 1.42 V vs NHE. This reduction potential implies that the S-H bond energy of CH3C(O)S-H is 88.6 kcal/mol (370.8 kJ/mol). The beta-fragmentation of the CH3C(O)S. radical, i.e., CH3C(O)S . --> CH3. + COS, was observed. Its kinetics was found to follow the Arrhenius equation, log(k(2)/s(-1)) = (12.3 +/- 0.1) - (10.1 +/-0.2)/theta, where theta = 2.3RT kcal/mol. At 22 degrees C, the CH3C(O)S . radical decays with a rate constant of 6.6 x 10(4) s(-1). The thermochemical properties of the CH3C(O)S radical and its beta-fragmentation reaction are compared with those of the benzoylthiyl radical (PhC(O)S .), as well as the corresponding oxygen counterparts, the acetyloxyl (CH3C(O)O .) and benzoyloxyl (PhC(O)O .) radicals.