The coil-to-globule transition of two poly (N-isopropylacrylamide) (PNI-PAM) ionomers with different ionic contents (0.8 and 4.5 mol %), but similar weight average molar masses, in deionized water was investigated by a combination of static and dynamic light scattering. In spite of the large difference in their ionic contents, both the ionomers have a nearly same lower critical solution temperature (LCST, similar to 32.5 degrees C). At temperatures higher than the LCST, the ionomer chains undergo a simultaneous intrachain coil-to-globule transition anti interchain aggregation to form nanoparticles thermodynamically stable in water. The average size of the nanoparticles decreases respectively as the ionic content increases and the ionomer concentration decreases. The interchain aggregation can be completely suppressed in an extremely dilute ionomer solution (< similar to 5 X 10(-6) g/mL), so that the intrachain coil-to-globule transition leads to the collapse of the ionomer chains into individual single-chain nanoparticles. Our results clearly indicate that there is a hysteresis in the coiling process (the globule-to-coil transition).