Platinum oxide (PtOx) nanoparticles (NPs) have been shown to possess anticancer activity by releasing ionic Pt species under biological conditions. However, the dissolution kinetics and the changes in the chemical state of Pt during PtOx dissolution have not yet been studied. To fill this gap, we prepared a composite (designated as PtOx@MMT-2) containing PtOx NPs on hollow mesoporous silica nanospheres and studied the dissolution of the material in different biorelevant media. We found that the release of Pt was retarded due to the adsorption of biomolecules on PtOx NPs during the degradation of host silica. The biomolecules adsorption also lowered the accessibility of PtOx NPs, resulting in the reduced catalase-like activity of the NPs. In line with the results, the cytotoxicity of PtOx@MMT-2, which was positively correlated to the amount of Pt uptake, was reduced by biomolecules adsorption. Our findings should be applicable to other metal (oxide) NPs under biological conditions and may provide implications for the design of nanomaterials for practical therapeutic applications. (C) 2019 Elsevier Inc. All rights reserved.