The absence of nonionic surfactant micelles in ultrafiltration membrane (molecular weight cut-off = 10,000) permeates is verified with the aid of a dynamic light-scattering (DLS) technique. DLS is also used to determine the hydrodynamic radii of micelles at concentrations above the critical micelle concentration. An empirical relationship between the micelle diameter, diffusion coefficient, and a pseudomolecular weight is plotted. The relationship can be used to screen high molecular weight cut-off membranes for surfactant-based UF applications.