The electrochemical current-voltage curve of a Si(111) surface measured in an aqueous solution is very sensitive to the surface condition. Using this property, we analyzed the Si(111) surface after chemical oxidation by H2O2. Thermodynamically, Si-Si back bonds of surface Si-H bonds are oxidized more easily than the Si-H bonds. However, when the surface was atomically fattened and hydrogen-terminated, the oxidation rate of the Si-Si back bonds was lowered and the surface Si-H bonds and Si-Si back bonds were oxidized simultaneously. The lowered reactivity of the Si-Si back bonds was attributed to the hindrance of the approach of H2O2 molecules to the Si-Si back bonds by the Si-H bonds regularly stretching perpendicular to the surface. Fourier transform infrared (FTIR) spectra showed that the Si-Si back bonds on step sites were oxidized very easily, and this was attributed to easy access of H2O2 molecules to the Si-Si bonds on the step sites. These results indicate that the regular structure of the atomically flattened Si(111) surface is the reason for its good stability against chemicals. (c) 2007 The Electrochemical Society.