The surface oxidation of hydrogen-terminated Si(111) single crystals in water has been studied by the adsorption of adenosinetriphosphate and chlorine from their aqueous solutions. The radioactive tracer (P-32) method and Auger electron spectroscopy have revealed that these anions are chemisorbed on the Si surface before the surface is fully oxidized. The results indicate that the silicon surface is positively charged in nearly neutral solutions, and is suggestive of the emergence of basic sites in the initial oxidation stage. Structural change of the surface-oxidized layer has been discussed in relation to the oxidation of a clean silicon surface under low oxygen pressure in a vacuum.