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 °C. Such formed particles are stable even at temperatures much higher than lower critical solution temperature (LCST ∼ 32 °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. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 749–755, 2010