The addition of mixture of polystyrene-b-poly(N-isopropylacrylamide) (PS-b-PNIPAM) and polystyrene homopolymer (h-PS) in tetrahydrofuran dropwise into water leads to nanoparticles with a PS core and a thermally sensitive PNIPAM shell. The effects of the ratio of the homopolymer to copolymer and temperature on the formation and stabilization of the dispersion were investigated by using a combination of static and dynamic laser light scattering. PNIPAM shell continuously collapses as temperature increases in the range 20-40 degrees C. Such formed particles are stable even at temperatures much higher than lower critical solution temperature (LCST similar to 32 degrees C) of PNIPAM. Our results reveal that the area occupied per hydrophilic PNIPAM chain on the hydrophobic PS core remains nearly a constant regardless of the amount of h-PS in the polymer mixture. This clearly indicates that the surface area occupied per hydrophilic group is a critical parameter for stabilizing particles dispersed in water. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 749-755, 2010