As a light-driven water-plastoquinone oxidoreductase, Photosystem II produces molecular oxygen as an enzymatic product. Additionally, under a variety of stress conditions, reactive oxygen species are produced at or near the active site for oxygen evolution. In this study, Fourier-transform ion cyclotron resonance mass spectrometry was used to identify oxidized amino acid residues located in several core Photosystem II proteins (D1, D2, CP43, and CP47) isolated from spinach Photosystem II membranes. While the majority of these oxidized residues (81%) are located on the oxygenated solvent-exposed surface of the complex, several residues on the CP43 protein ((354)E, (355)T, (356)M, and (357)R) which are in close proximity (<15 Å) to the Mn(4)CaO(5) active site are also modified. These residues appear to be associated with putative oxygen/reactive oxygen species exit channel(s) in the photosystem. These results are discussed within the context of a number of computational studies which have identified putative oxygen channels within the photosystem.