The effect of urea on micelle formation and the microenvironmental properties of sodium dodecylbenzenesulfonate micellar solutions has been investigated by using electron spin resonance spectroscopy and surface tension measurement at the air/water interface. Two different nonionic spin probes, 5-doxylstearic acid and the piperidinyl-1-oxy with long hydrocarbon chains (Tempo, C-6-Tempo, C-12-Tempo, C-16-Tempo), have been used for studying sodium dodecylbenzenesulfonate solutions as a function of surfactant and urea concentrations. The surface tension results show that the addition of urea increases the critical micelle concentration values of the surfactant. The analysis of the nitrogen hyperfine coupling constant (A(N)) and the correlation time (tau) for the probe motion indicates that urea molecules interact with the polar headgroups of the sodium dodecylbenzenesulfonate micelles and penetrates below the polar headgroups. The addition of urea slightly decreases the micropolarity and strongly increases the microviscosity of the micellar interface. These results are in agreement with the recently reported mechanism where urea molecules replace some water molecules that solvate the hydrophobic chain and the polar headgroups of the surfactant.