Mixed micelles of polystyrene-b-poly(N-isopropylacrylamide) (PS-b-PNIPAM) and two polystyrene-b-poly(ethylene oxide) diblock copolymers (PS-b-PEO) with different chain lengths of polystyrene in aqueous solution were prepared by adding the tetrahydrofuran solutions dropwise into an excess of water. The formation and stabilization of the resultant mixed micelles were characterized by using a combination of static and dynamic light scattering. Increasing the initial concentration of PS-b-PEO in THF led to a decrease in the size and the weight average molar mass (< M-w >) of the mixed micelles when the initial concentration of PS-b-PNIPAM was kept as 1 x 10(-3) g/mL. The PS-b-PEO with shorter PS block has a. more pronounced effect on the change of the size and < M-W > than that with longer PS block. The number of PS-b-PNIPAM in each mixed micelle decreased with the addition of PS-b-PEO. The average hydrodynamic radius < R-h > and average radius of gyration < R-g > of pure PS-b-PNIPAM and mixed micelles gradually decreased with the increase in the temperature. Both the pure micelles and mixed micelles were stable in the temperature range of 18 degrees C-39 degrees C. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1168-1174, 2010