Peroxiredoxin 2 (Prx2) is a redox enzyme that is abundantly expressed in red blood cells (RBCs) and has been the focus of clinical attention for monitoring the oxidative status. We previously developed a method to quantify the reduced and hyperoxidized forms of Prx2 in human RBCs using reverse-phase high-performance liquid chromatography (HPLC). In the present study, we investigated the hyperoxidative status of Prx2 at the molecular level in a post-translational modification analysis using a liquid chromatography-tandem mass spectrometry (LC-MS/MS) system. The LC-MS/MS analysis of the trypsin digests of Prx2 fractionated by reverse-phase HPLC demonstrated that the cysteine-51 residue (Cys-51) of the protein was modified with the hyperoxidative functional groups, sulfinic acid (-SOH) and sulfonic acid (-SOH), in RBCs treated with tert-butyl hydroperoxide (t-BHP). Furthermore, a selected ion monitoring (SIM) analysis quantitatively showed that sulfinic acid- and sulfonic acid-induced modifications in Prx2 Cys-51 were increased by the treatment with the oxidant. It was demonstrated that the peroxidatic cysteine of Prx2 separated using our HPLC system for oxidative monitoring was hyperoxidized into sulfinic acid and sulfonic acid in RBCs under an oxidative stress condition.