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Because of the rich polymorphs and lower diffusion energy barriers of copper chalcogenide systems, the phase transformation of colloidal Cu_2–xS (0 ≤ x ≤ 1) nanocrystals is critical for understanding their fundamental properties and designing convenient synthetic routes. In this work, high quality digenite Cu_1.8S nanocrystals with rhombohedral structure were synthesized at gram-scale. The as-prepared colloidal nanocrystals undergo an in situ phase transformation from rhombohedral Cu_1.8S nanocrystals to hexagonal CuS clusters upon keeping the resulting colloidal solution for a few days. The observed transformation was explored by a combination of structural and spectroscopic analyses, including powder X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. A possible mechanism is proposed and thoroughly discussed …