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The mechanisms of bleaching of discolored coronal teeth using hydrogen peroxide (H2O2) were investigated. In a scanning–electron-microscopy study, the intertubular dentin and peritubular dentin were dissolved by high concentrations of H2O2, which is used for bleaching. The X-ray diffraction study showed that hydroxyapatite was not influenced by H2O2. In an electron–spin-resonance study, more hydroxyl radical (· OH) was detected as the H2O2 concentration was increased. When amino acids that are core components of dentin proteins, such as proline and alanine, were added to H2O2, the generation of · OH decreased, but there was no change when glycine was added. A nuclear–magnetic-resonance study showed that proline was degraded completely by H2O2, the structure of alanine changed slightly, and glycine was not affected by H2O2. It is suggested that H2O2 and · OH do not influence the inorganic tissue of dentin but attack the organic component of dentin. These facts suggest that · OH has the main role in tooth bleaching with H2O2.