Thermosensitive graft copolymers of N-vinylcaprolactam and poly(ethylene oxide)-alkyl methacrylate macromonomer were synthesised with different grafting densities. Association of the polymers in dilute aqueous Solutions was studied below and above the LCST using static and dynamic light scattering, microcalorimetry, pressure perturbation calorimetry (PPC), and fluorescence probe experiments. Owing to the amphiphilicity of the grafts, the most densely grafted polymer forms intrapolymeric structures while the less grafted polymers build up mixtures of intra- and interpolymeric associates at temperatures below LCST. These assemblies solubilise hydrophobic substances such as pyrene. Upon heating, the graft copolymers aggregate in water and form nano-sized particles, mesoglobules. Heat induced particles are exceptionally stable against further aggregation, and the dilution of the Solutions has no effect on the particle size or shape. The structure of the particles can be frozen by physically crosslinking the collapsed polymers with a phenol. e.g. 1,2-benzenediol. (c) 2005 Elsevier Ltd. All rights reserved.