Cerium dioxide nanoparticles have been proposed for an increasing number of applications in biomedicine, cosmetic, as polishing materials and also as byproducts from automotive fuel additives. The aim of this study was to examine the potential in vitro cyto- and genotoxicity of nano-sized CeO₂ (7 nm) on human dermal fibroblasts. By combining a physico-chemical and a (geno)toxicological approach, we defined the causal mechanisms linking the physico-chemical properties of nano-CeO₂ with their biological effects. Using X-ray absorption spectroscopy, we observed a reduction of 21±4% of the Ce⁴⁺ atoms localized at the surface of CeO₂ nanoparticles due to the interactions with organic molecules present in biological media. These particles induced strong DNA lesions and chromosome damage related to an oxidative stress. These genotoxic effects occurred at very low doses, which highlighted the importance of a genotoxicological approach during the assessment of the toxicity of nanoparticles.