Copolymers of N-isopropyl acrylamide, 2-acrylamido-2-methylpropane sulfonic acid and an acrylic ester of a nitroxide radical have been synthesized by radical polymerization. The products are polymers where the nitroxide side group is reversibly protonated by the sulfonic acid groups. The lower critical solution temperature (LCST) and the amount and size of the aggregates formed in aqueous solutions above the LCST are dependent not only on the amount of sulfonic acid groups in the polymer, but also on the pH and ionic strength of the solvent. Electron paramagnetic resonance spectra of the labelled polymers are sensitive to the rate and amplitude of the segmental motion of the polymer chain, and also to the rate of proton exchange between the sulfonic acid and the nitroxide.