The understanding of the hydrolysis mechanism of lanthanide ions is limited by their elusive coordination configuration and undeveloped technology. A potential solution by high-resolution mass spectroscopy studies is hindered by the lack of a stable model under electrospray ionization (ESI) conditions and the complexity of the spectra. Herein, it is demonstrated that diketonate ligands can efficiently stabilize the hydrolyzed intermediate cluster of Ln3+ under ESI conditions, and an effective mass difference fingerprint of isomorphism (MDFI) method is proposed, which can allow the determination of the nuclearity-number of the species without depth resolution. Thus, the hydrolysis of Ln3+ into an atomically precise hydroxide cluster is observed at the level of precise formulae. The species evolution upon hydrolysis is along the dominant path of {Eu3}-{Eu4}-{Eu9}-{Eu10}-{Eu11}-{Eu15}-{Eu16} and a nondominant path of {Eu3}-{Eu4}-{Eu8-1}-{Eu8-2} under the investigated conditions. The crystal of the {Eu16} species was obtained via low-temperature crystallization, and single-crystal X-ray diffraction studies show that its structure contains three octahedral {o-Ln6} units. The contradiction between multiple {o-Ln6} units in the structure and the absence in the formation process indicates that the repetitive subunit observed in the structure does not necessarily correspond to the construction units of high-nuclearity clusters. Photophysical measurements indicate that Eu16 cluster has a high total emission quantum efficacy of 12.8% in the solid state. This study provides fundamental insights into the formation, evolution, and assembly of small lanthanide hydroxide units upon hydrolysis, which is vital for the goal of directional synthesis of lanthanide hydroxide clusters.