Thermoresponsive hydrogel nanoparticles were prepared by self-assembly of two different hydrophobically modified polymers, namely a cholesterol-bearing pullulan (CHP) and a copolymer of N-isopropylacrylamide (NIPAM) and N-[4-( 1-pyrenyl)butyl]-N-n-octadecylacrylamide (PNIPAM-C18Py), The interactions between CHP and PNIPAM-C18Py were investigated by fluorescence spectroscopy, dynamic light scattering, and size exclusion chromatography. After ultrasonication of a mixture of CHP and PNIPAM-C18Py (5:1 by weight) at 25 degrees C, monodisperse nanoparticles (Dh = 45 nm) were obtained, consisting of self-assembly of the two polymers associated via their hydrophobic moieties. Evidence from fluorescence and dynamic light scattering demonstrated that, above 32 degrees C, the lower critical solution temperature (LCST) of PNIPAM-C18Py, the colloidal mixed nanoparticles increase in diameter (from 47 to 160 nm), but no macroscopic aggregation could be detected. This phenomenon was thermoreversible: upon cooling the particles recovered their original diameter.