Early stages of polymeric nanoparticle formation are monitored by in situ synchrotron small-angle X-ray scattering (SAXS) experiments. A new strategy enables the preparation of stable polymeric nanoparticles from homopolymers (not copolymers) of one type only and without any assembly-triggering additives. Poly(ethylacrylic acid) (PEA) in an aqueous solution is tuned to the pH, where it exhibits thermosensitivity and the formation of nanoparticles is temperature-induced. The kinetics of this formation are clearly monitored by SAXS. Experiments indicate the slightly elongated, not exactly spherical shape of the resulting nanoparticles, which is also confirmed by cryo-transmission electron microscopy (cryo-TEM). Quasielastic neutron scattering (QENS) shows that the PEA nanoparticles are relatively loose with a lot of solvent inside, in agreement with results from previous light scattering work.