Stability of the styrene miniemulsion costabilized by carboxylated polyurethane resins was investigated. The shelf life and the droplet size were measured. The results show that styrene miniemulsion costabilized by carboxylated polyurethane can be prepared although it is not as efficient as is hexadecane. The shelf life of the miniemulsion is influenced by the amount of the polyurethane and sodium dodecyl sulfate. The alkaline condition and the smaller molecular weight of the polyurethane are in favor of the stability of the miniemulsion. The polymerization results show that the hydrophility of the particle or droplet surface plays an important role in the process of the nucleation and polymerization. Homogeneous nucleation and droplet nucleation coexist in these systems. Nucleation lasts to 40-60% conversion of the monomer in the acid medium, but it continues until the end of the polymerization in the alkaline medium. (C) 2003 Wiley Periodicals, Inc.