The association of poly(ethylene oxide-b-methacrylic acid) and poly(amidoamine) dendrimers was examined by dynamic light scattering and small angle neutron scattering. With increasing amounts of the G4 dendrimer as the counterion, the size of the assemblies increases until it reaches a hydrodynamic radius of about 70 nm. The structure is consistent with poly(methyl methacrylate) (PMAA) chains closely aggregating with the dendrimers at low dendrimer amounts and volume-filling PMAA blocks at higher dendrimer contents. Similar behavior was observed for G4 and G2 dendrimers, while smaller G0 molecules showed an opposite dependence. The results represent an example of finite size assemblies formed by "electrostatic self-assembly" that are stable in aqueous solution and represent equilibrium structures, the structure and size of which can be tuned through the building units, loading ratio, and pH.