Aqueous dispersions of hydrophobic high-molar-mass cellulose-graft-polystyrene (Cel-g-PS) copolymers were prepared by nanoprecipitation from 1,4-dioxane solutions using sodium dodecyl sulfate as a stabilizer. The size and structure of formed Cel-g-PS nanoparticles were studied by scattering techniques (static and dynamic light scattering, SAXS) and transmission electron microscopy. The scattering and microscopy data show that nanoprecipitation of Cel-g-PS leads to the formation of polydisperse aggregates of collapsed Cel-g-PS macromolecules containing entrapped SDS micelles in the kinetically frozen state. The final size of the aggregates can be tuned by changing the copolymer concentration, c P , in the dioxane solution. The average hydrodynamic radius of the aggregates, R H , scales as [(R H /R H,0 ) 2.5 –1]∼c P , where R H,0 is the R H value extrapolated to the zero copolymer concentration.