Hydrogen-abstraction reactions of various radicals with 4-mercaptobenzoic acid (MB) to produce the 4-carboxyphenylthiyl radical have been studied by pulse radiolysis in aqueous solutions and in the ionic liquid methyltributylammonium bis(trifluoromethylsulfonyl)imide (R4NNTf2). The rate constants in aqueous solutions are in the range of (1-3) x 10(8) L mol(-1) s(-1) for the reactions of MB with several alkyl radicals, are higher with reducing radicals (6.4 x 10(8) L mol(-1) s(-1) for CH3CHOH and 1.4 x 10(9) L mol(-1) s(-1) for (CH3)(2)(COH) and lower with oxidizing radicals (less than or equal to 10(7) L mol(-1) s(-1) for (CH2COCH3)-C-.). Because the bond dissociation energy for the S-H bond is much lower than that for the C-H bonds involved in these reactions, it appears that hydrogen abstraction from mercaptobenzoic acid is not controlled by the relative bond dissociation energies but rather by the electron density at the radical site through a polar transition state. The rate constants for similar reactions in alcohols are slightly lower than those in water, supporting a polar transition state. The rate constants in the ionic liquid are in the range of 10(7)-10(8) L mol(-1) s(-1) and are essentially controlled by the diffusion rate; variations within this range appear to be due mainly to changes, in viscosity, The (.)CF3 radical reacts slightly more slowly (3.6 x 10(6) L mol(-1) s(-1)) with MB in the ionic liquid, in agreement with the low reactivity in water of radicals bearing electron-withdrawing groups.