A fluorinated thermal SiO2, grown after HF surface treatment without de‐ionized water rinse, was estimated to contain ∼3×1013cm−2of fluorine by the19F(p,&agr;&ggr;)16O reaction. Secondary‐ion mass spectrometry data indicate that the SiF distribution is peaked at the SiO2/Si interface in the fluorinated oxide. The time‐dependent change of the absolute amount of fluorine on the HF‐treated silicon surface as a function of storage time in air or in vacuum was also investigated by the19F(p,&agr;&ggr;)16O reaction. The initial number of fluorine atoms on the HF‐treated silicon surface was estimated to be ∼1015cm−2before substantial desorption took place. Fluorine atoms desorb from the silicon surface much more rapidly if the sample is stored in air than in vacuum. These results were also supported by the x‐ray photoelectron spectroscopy measurement.