The conventional synthesis of cerium oxide nanoparticles (CeNPs) relies on the use of trivalent cerium and an oxidizer to ensure a thorough hydrolysis. This study explores CeNPs formation via the hydrolysis of tetravalent cerium, specifically through the spontaneous reaction of ceric ammonium nitrate (Ce(NH4)2(NO3)6) under varied aging conditions, including adjustments in pH, concentration, and heating strategy. This research adopts a thermo-hydrolysis method to synthesize CeNPs with small primary particles (2–3 nm) to loose dendritic clusters (5–8 nm). Additionally, when the starting pH of the solution decreases below 0.8, it favors the formation of clusters with larger diameters (15–20 nm). Analyses using dynamic light scattering (DLS), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) indicate that CeNPs particle size in acidic environments is predominantly determined by the extent of primary particle aggregation, influenced by nitrate adsorption. Overall, our comprehensive analysis of the CeNPs evolution provides fundamental insights for the development of a size/structure controllable synthetic strategy for nanoparticle manufacturing.