Polymeric nanoparticles were formed when a carboxylated polystyrene solution in tetrahydrofuran was added dropwise into water. The hydrophobic association of the chain backbones was balanced by the hydrophilic repulsion of the carboxylic groups. The resultant particles were very stable in water and resembled those formed in surfactant-free emulsion copolymerization. The formation and stabilization of such particles were studied by a combination of static and dynamic laser light scattering. The particle size decreases as the carboxylation extent [COOH] increases in the range 7-26 mol %. Further increase of [COOH] leads to an increase of the particle size. On the other hand, as [COOH] increases, the monotonic decrease of the average particle density [p] and the increase of the ratio of the average radius of gyration to the average hydrodynamic radius ([R-g]//[R-h]) reveal the swelling of the particles, indicating a gradual change of their structure from spherelike to clusterlike. Using longer polymer chains or more ionizable counterions results in smaller particles. Our results indicate that the formation and stabilization are controlled by a delicate balance between hydrophobic and hydrophilic interactions. More hydrophilic groups stabilizing a larger total interfacial area generally lead to smaller particles.