The application of the time-resolved fluorescence quenching method (TRFQ) to study the structure and aggregation behavior of nonionic surfactant polyoxyethylene(4)lauryl ether (C(12)E(4)) in reverse micellar systems is presented. Solutions of different concentrations in three nonpolar solvents (cyclohexane, n-decane, and n-dodecane), at different water content and temperature, employing Ru(bpy)(3)(2+)-methylviologen, solubilized in the polar core of micelles, as probe-quencher pair, were studied. The aggregation number and fluorescence quenching rates were determined and the size and shape of micelles estimated; the dependence of their values on medium was evidenced. Dynamic light scattering was utilized as a complementary method in checking the hypotheses regarding micellar size/shape. It was found that in cyclohexane, at the compositions studied, the micelles can be regarded as spherical, growing with the water content, with a constant area per head group of about 50 Angstrom(2). Only at the lowest water content (3 water molecules per surfactant or less) is there a deviation with a larger area per surfactant. The radius of the polar core is then smaller than the length of the EO tail, and there is no free water that could allow the micelle to swell beyond this limit. The micelles in decane and dodecane appear to grow in a nonspherical way and become much larger with water addition.