The effect of phosphate on the ability of Ce-containing bioactive glasses to inhibit oxidative stress was studied on compositions based on Hench (46.2%SiO224.3%Na2O26.9%CaO2.6P2O5, mol%) and Kokubo (50.0%SiO225.0%Na2O25.0%CaO) glasses. In particular, the reduction of catalase mimetic activity of Ce-containing glasses due to the presence: i) of P2O5 in the glass compositions, and ii) of phosphate groups in the solution employed for catalase mimetic activity tests was explained and rationalized by combining SEM, XPS, XRD, DTA, FT-IR and UV-vis experiments with Molecular Dynamics simulations.The results suggest that the Ce ions play a different structural role in the two series of glasses. In particular, in phosphate free glasses Ce is coordinated by non-bridging oxygens (NBOs) originated from the disruption of the silicate network, whereas in phosphate containing glasses the NBOs around Ce ions belong to orthophosphate groups. The latter groups stabilize the Ce3+ species subtracting them from the interconversion process between Ce3+ and Ce4+, which is of fundamental importance for the exhibition of the catalase mimetic activity

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