Poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-AA) ionomer chains can form stable nanoparticles in water at temperatures higher than their lower critical solution-temperature. A combination of the weight average molar mass (M-w) from the absolute average scattering intensity and the hydrodynamic radius distribution (f(R-h)) from the line width distribution G(T) was used to study the influence of the AA content and molar mass of the ionomer chains on the formation and structure of these novel surfactant-free nanoparticles. Our results reveal (1) particle size decreases as the AA content increases; (2) the particle formation is not only thermodynamically controlled but also dependent on the formation temperature, the polymer concentration, and the molar mass of the ionomer chains, because it involves simultaneously the process of the intrachain collapse and interchain aggregation; and (3) the hydrodynamic density ([rho]) of the particle slightly increases as the formation temperature increases hut remains to be a constant when the ionomer concentration varies. We also found that the weight average particle mass (M-w,M-particle) can be scaled with the ionomer concentration (C) as M-w,M-particle proportional to C-2/3, revealing that the aggregation is a diffusion-controlled process.