Bound states of counterions during the coil-globule transition of poly (acrylic acid) in water/organic solvent mixtures were investigated by NMR spectroscopy of alkali metal cations (Li+, Na+, Cs+). Accompanying the transition, the line widths of the respective NMR peaks significantly increased with increasing the organic solvent composition in the medium. Although this line width broadening suggests that some specific counterion binding with desolvation is involved with the coil-globule transition, the most marked broadening was observed in higher organic solvent compositions than those of the coil-globule transition region detected by the viscometry. Namely, the specific counterion binding with desolvation proceeds even after the polymer chain collapsed. This means in turn that such a strong counterion binding is not a prerequisite for the coil-globule transition, at least at the stage of the onset. For the Li+/Cs+ mixed counterion system in 60 vol % DMSO, where our previous conductivity data suggested that the specific counterion binding occurred only for Cs+ during the coil-globule transition induced on mixing with Li+, a significant increase in the line width was also observed only for Cs+. The coincidence between the conductivity and the NMR results for the Li+/Cs+ mixed counterion system strongly supports a working hypothesis, "size-fitting effect," that has been proposed to determine the counterion specificity observed for the conformational transitions of polyelectrolytes. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2132-2139, 2009