Temperature- and pH-controlled association of terpolymers of N-isopropylacrylamide (NIPA) with I-vinylimidazole (VI) and polyethylene glycol (PEG) has been investigated by light scattering and atomic force microscopy (AFM) in situ. The polymers contained 0-15 mol% VI and 0-2 mol% PEG. The phase transition temperatures (T-p) have been in the range of 32-45 degrees C and exhibited significant dependence on the pH of solution in the pH range between 5 and 8. The T-p of the polymers increased with increasing VI content and with decreasing pH, confirming major effect of VI ionization status on T-p. The presence of PEG grafts in the polymer structure had augmenting effect on the magnitude of pH-responsiveness and on the pH-independent colloidal stability of the polymer particles formed above T-p. Incorporation of VI into the polymer structure had similar, but pH-dependent effect on colloidal stabilization of the polymer particles. The size of the particles formed after the phase transition is driven by the association of the collapsed NIPA segments in the globule conformation and it decreased with decreasing pH. The phase transition temperature of the polymers could be adjusted to increase from temperatures below, to temperatures above body temperature upon decreasing pH from 7 to 6, suggesting that such polymers could provide a material platform for a variety of biomedical applications. AFM analysis in situ showed a fully reversible formation of particles in the solutions of the polymers above their T-p. (c) 2005 Elsevier Ltd. All rights reserved.