Hyperfine coupling constants and line widths of neutral 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1 -oxyl, cationic 3-ammonium-2,2,5,5-tetramethylpyrrolidine-l-oxyl, and anionic potassium 2,2,5,5-tetramethyl-l-pyrrolidinyloxy-3-carboxylate spin probes have been determined in water, in benzene, and in 0.10 M dodecylammonium propionate (DAP) solubilized water in benzene by electron paramagnetic resonance (EPR) spectroscopy. The neutral probe is somewhat soluble in benzene and exchanges among the environments provided by the reversed micelles. The exchange frequency is, however, ≥6ËŸ106 s-1. Conversely, both charged spin probes favorably partition into water and their EPR data provide information on the microenvi-ronments of these probes. The anionic radical is likely to be anchored at the water-hydrocarbon interface with the polar carboxylate group intruding into the surfactant solubilized water pool. The cationic radical, however, is situated in a less polar region, corresponding to that occupied by the ammonium head group of the surfactant. The microviscosities reported by the charged probes in 0.10 M DAP in benzene increase initially with increasing water concentration until maxima are reached at 0.40 M cosolubilized water concentration, when free water molecules begin to appear. Important consequences of these results are the required different environments for the surfactant cation head group and its counterion as well as the presence of more than one type of bound water molecule. The rate of quenching of the anionic radical by phenylhydrazine remained unaffected by micellar DAP but that by sodium ascorbate decreased if the concentration of the quencher is taken to be that dissolved in the surfactant solubilized water pool.