Stable aqueous dispersions of nanoparticles were prepared by polyelectrolyte complex formation between well-defined poly(ethylene glycol)-block-poly(2-acrylamido-2-methyl-1-propane sodium sulfonate) and poly(ethylene glycol)-block-poly[2-(dimethylamino)ethyl methacrylate] diblock copolymers. Controlled synthesis of the copolymers was achieved by water-based atom transfer radical polymerization (ATRP). The nanoparticles were characterized by a quite narrow and monomodal size distribution as evidenced by dynamic light scattering (DLS) and confirmed by atomic force microscopy (AFM) after solution casting and freeze-drying.