Results are presented for anin situsynchrotron-based ultra-small-angle X-ray scattering (USAXS) study of the solution-mediated formation and growth of nanocrystalline ceria (n-CeO2) using a new remote-controlled, isothermal, circulating fluid flow cell. The fluid flow mitigates or reduces X-ray beam-induced damage, air bubbles or particulate flocculation within the bulk solution, but prevents any coarse particulates that do form from settling out from suspension. Combined with the large-scale range accessible in USAXS studies, the flow cell has enabled measurement,in situand in real time, of structural characteristics from 10 Å to a few micrometres in size as a function of the changing physical and chemical conditions. By applying a multi-component model, the nanoparticle formation and growth component has been identified. Control and online monitoring of flow rate, temperature and pH suspension conditions have permitted real-time studies of the formation and growth of the individual n-CeO2particles from homogeneous dilute solution over several hours. Aspects of the nanoparticle nucleation and growth are revealed that have not been observed directly in measurements on this system.