Nonionic poly(oxyethylene) surfactants with four ethylene oxide units (i.e. monodisperse poly(oxyethylene) [4] lauryl ether (C(12)E(4)), Brij 30, and poly(oxyethylene) [4] nonylphenol (NPE(4))) have been studied in reverse micellar systems with two nonpolar solvents (cyclohexane and decane), at different concentrations, water contents, and temperatures using two spin probes, 4-[N,N-dimethyl-N-(n-butyl)ammonio]-2,2,6,6-tetramethylpiperidine-1-oxyl bromide (I) and 5-doxyl stearic acid (II), and two fluorescence probes, 1-anilinonaphtalene-8-sulfonic acid (III) and 1-pyrenesulfonic acid sodium salt (IV). Micellar radii lower than 20 Angstrom have been found for "dry" micelles from rotational correlation times of I, as determined by ESR, and from the fluorescence anisotropy decay of III, The study has focused on the determination of water distribution in the core by using the isotropic nitrogen hyperfine splitting constant, alpha(N), and the rotational correlation time tau(c), of the spin probe I along with fluorescence band maxima shifts of III and band intensity ratios of IV, all data being calibrated vs the corresponding values in homogeneous poly(oxyethylene) (POE)/water mixtures with varying water proportion. A measure of the water segregation in the polar core, with respect to the reference POE/water mixtures, is proposed. The results clearly indicate a low hydration degree of the POE chains with the resulting segregation of water in the polar core. The effect increases with water concentration and is more pronounced with cyclohexane, as compared to decane, as solvent. Order degrees of the surfactant chains in the core have been determined from the components of the A tenser of probe II. The effects of increasing water content and temperature were sought. For comparison, lamellar structures have been also investigated. Differences between micellar and lamellar structures, regarding the water segregation and the dependence of order degree on water content and temperature, were noticed. These seem to rule out planar structures (lamella, hanks) and to favor curved ones for the micelles in the systems investigated.