Nanoparticles (NPs) are increasingly being incorporated into cosmetic formulations, and NP-containing sunscreens currently contain zinc oxide or titanium dioxide, but not cerium dioxide (CeO2). Nanoparticulate CeO2 (nanoceria) has antioxidant properties and we have previously shown its in vitro protection of human skin cells from harmful UV radiation. However it is unclear if this effect occurs via its antioxidant or UV absorbing properties. Therefore, we investigated the antioxidant and UV protective effects of nanocerias in both cellular and cell-free systems. Immortalised human keratinocytes (HaCaT) cells were exposed in vitro to OECD test nanomaterials (NM) at 0.01-1 mg/mL in the following forms: small pristine “NM-211” (Antaria; 5-10 nm), medium pristine “NM-212” (Umicore; 50-70 nm) or bulk pristine “NM-213” (Sigma-Aldrich; >100 nm). Particulate exposures were performed in the absence (“sham”) or presence of UV radiation at 0.3x minimal erythemal dose (UVA, 12 kJ/m2 and UVB, 0.6 kJ/m2). Superoxide levels (MitoSOX red assay) and cell viability (MTS assay), were determined 24 hours post-irradiation. In sham-irradiated HaCaT cells, nanoceria at 1 mg/mL increased superoxide production by 40-60% (NM-212 > NM-211) compared to a 25% increase with bulk ceria. UVAB radiation alone doubled superoxide levels, but prior addition of nanoceria (and not bulk ceria) reduced superoxide production to sham-irradiated levels (NM-211 > NM-212). There was an inverse correlation with cell viability, where nanocerias at ≥1 mg/mL were more cytotoxic than bulk ceria in sham-irradiated cells, but conferred protection in UV-irradiated cells at between 0.1 and 1 mg/mL (NM-212 > NM-211; NM-213=no effect). When nanocerias were added post-UV radiation, their photoprotective effect was abrogated. Also, nanocerias did not scavenge superoxide radicals generated in cell-free systems (phenazine methosulfate/NADH or xanthine/xanthine oxidase systems). These findings suggest that nanoceria’s protective mechanism relates to its UV absorbing capacity rather than its antioxidant effect.