We have found that silicon shows a large anodic current and a high etching rate in ammonium fluoride solutions at a pH of approximately 6.4. The reason for the unique pH dependencies was investigated by analyzing silicon surfaces after treatments with ammonium fluoride solutions at various pHs. Zeta potentials of the surfaces treated with-a solution at a pH of approximately 6.4 were different from those treated at other pHs, suggesting a high reactivity in solutions in this pH region with silicon surfaces. X-ray photoelectron spectra of silicon showed that the amount of fluorine atoms bound to the surface was largest after treatment with a solution at a pH 6.4. These results are discussed in connection with the unique electrochemical and etching behavior of silicon in aqueous solutions of ammonium fluoride.