Polystyrene-b-poly(acrylic acid) block copolymers, prepared by sequential anionic polymerization, were found to associate to form spherical, water soluble micelle-like aggregates. The micelle core radii were measured by transmission electron microscopy(TEM). It was found that the core radii scaled as r(core) similar to (NBNA-0.15)-N-0.4 (where N-B and N-A are the lengths of the insoluble and soluble blocks, respectively), illustrating that the soluble block exerted a considerable influence on the colloidal structure of the micelles. The micellar solutions were also studied by dynamic light scattering (DLS), yielding values of the micelle hydrodynamic radius, r(h). These studies showed that, in the absence of salt, the ionized form of the poly(acrylic acid) chains in the coronal layer of the micelles had a highly extended conformation.