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Even though the plant uptake of cerium oxide nanoparticles (CeO₂ NPs) has been reported, the mechanisms remain unknown. This study aimed to provide new insights into CeO₂ NPs plant uptake through two objectives: (1) to investigate whether CeO₂ NPs dissolute before their plant uptake and (2) to determine the in-planta speciation of Ce. Bench scale experiments were conducted by growing radish in solutions containing 10 mg elemental Ce/L of bulk CeO₂ particles, CeO₂ NPs or ionic Ce. Transmission electron microscope and inductively coupled plasma-mass spectrometry (ICP-MS) analysis suggested that one pathway for CeO₂ NPs uptake was through direct uptake of intact CeO₂ NPs. More importantly, our results confirmed that part of the particulate CeO₂ was transformed into ionic Ce on the root surface before they were taken up by plants. Ionic Ce uptake and transport was a primary mechanism for Ce …