The one-electron reduction of methanesulfonyl chloride (MeSO2Cl) leads, in the first instance, to an electron adduct MeSO2Cl center dot- which lives long enough for direct detection and decays into sulfonyl radicals MeSO2 center dot and Cl-, with k = 1.5 x 10(6) s(-1). Both, MeSO2Cl center dot- and MeSO2 center dot showed a similar absorption in the UV with lambda(max) of 320 nm. In the presence of oxygen, MeSO2Cl center dot- transfers an electron to O-2 and establishes an equilibrium with superoxide. The rate constant for the forward reaction was measured to 4.1 x 10(9) M-1 s(-1), while for the back reaction only an interval of 1.7 x 10(5) to 1.7 x 10(6) M-1 s(-1) could be estimated, with a somewhat higher degree of confidence for the lower value. This corresponds to an equilibrium constant in the range of 2.4 x 10(3) to 2.4 x 10(4). With reference to E degrees (O-2/O-2(center dot-)) = -155 mV, the redox potential of the sulfonyl chloride couple, E degrees(MeSO2Cl/MeSO2Cl center dot-), thus results between being equal to -355 and -414 mV (vs NHE). MeSO2Cl center dot- reduces (besides O-2) 4-nitroacetophenone. The underlying electron transfer took place with k = 1.5 x 10(9) M-1 s(-1), corroborating an E degrees for the sulfonyl chloride couple significantly exceeding the above listed lower value. The MeSO2 center dot radical added to oxygen with a rate constant of 1.1 x 10(9) M-1 s(-1). Re-dissociation of O-2 from MeSO2OO center dot occurred only very slowly, if at all, that is, with k < 10(5) s(-1). MeSO2 center dot radicals can act as the catalyst for the cis-trans isomerization of several Z- and E-mono-unsaturated fatty acid methyl esters in homogeneous solution. The effectiveness of the isomerization processes has been addressed, and in the presence of oxygen the isomerization is completely suppressed.