Dynamic light scattering and capillary viscometry have been employed to study aqueous micellar solutions of nonionic (pentaoxyethylene dodecyl ether) and ionic (decyl-, dodecyl-, and tetradecyltrimethyammonium bromides) surfactants in a wide concentration range covering the first and second critical micelle concentrations (CMC1 and CMC2, respectively). The concentration curve for the diffusion coefficient of the nonionic surfactant has been shown to correspond to a monotonically decreasing function. The diffusion coefficients of the ionic surfactants pass through maxima above CMC1. As the alkyl chain length increases, the slopes of the concentration curves rise in the range of the linear growth in the diffusion coefficient, while the values of the maxima increase, their positions shifting toward lower concentrations. The concentration dependences of the diffusion coefficients have been explained based on their relation to activity coefficients.