The influence of an adsorbed layer of a surfactant on the flow behavior of a latex is considered. The system studied consists of a poly(styrene) core latex without chemically bound charges (diameter : 146 nm) and a layer of poly(ethylene oxide) chains (length : 80 ethylene oxide units) affixed to the surface of the particles through adsorption of the surfactant Lutensol AT80 (C16-18EO80). Both the core latex as well as the latex covered by surfactant have been studied by small-angle X-ray scattering (SAXS). The hydrodynamic thickness of the layer was determined by rheology to be 11.7 nm whereas SAXS gives an extension of the layer of approximately 12 nm. The result demonstrates that realistic hydrodynamic radii of latex particles result from viscosimetric measurements. Relative viscosities measured at higher Volume fractions compare favorable with a recent experimental master curve proposed for suspensions of hard spheres (Meeker, S. P.; Poon, W. C. K.; Pusey, P. N. Phys. Rev. E 1997, 55, 5718).