A spin label was attached to poly(N4sopropylacrylamide) (PNIPAM) to allow EPR spectroscopy to be used as a probe of the interactions between the solvent and this polymer in water, methanol, and watermethanol mixtures. Labeled polymers were prepared by reaction of a copolymer of N-isopropylacrylamide and N-(acry1oxy)succinimide with 4-amino-2,2,6,6-tetramethylpiperidine 1-oxide. The label contents of the polymers (M, 2.9 X lo6) were 1 X 10" and 1.6 X lo4 mol g*. EPR spectra of solutions of the labeled polymers (3 g L-9 were recorded (a) at constant temperature as a function of solvent composition and (b) for several water-methanol mixtures as a function of temperature (-10 to +35 O C ) . The spectra were analyzed in terms of the isotropic hyperfine coupling constant and the correlation time for the reorientation motion. The temperature and composition dependence of the parameters was determined by fitting the line shapes of the experimental spectra to simulated spectra. The results support a model involving preferential adsorption of methanol to the polymer chains in mixed methanol-water solutions, as the main contributor to the cononsolvency phenomenon.