The time depending influence of HF‐H2O2mixtures on the Si(100) and Si(111) surface was studied by means of x‐ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy (HREELS). The H2O2concentration was varied in the range between 0% and 30%, for the HF concentration 0.5% was used. The oxygen coverage of the silicon surface increases with H2O2concentration which shows up in HREELS spectra as Si–O–Si bridge bonded oxygen and O–H groups. Oxidation of Si–Si backbonds proceeds as can be seen by changes of the Si–H stretching mode. The oxygen uptake of Si(100) with a substantial higher amount of Si–O–Si is faster as compared with Si(111) which reveals Si–OH. Further on we find changes in the morphology of Si(100) surfaces. The dihydride termination decreases with increasing H2O2concentration and immersion time which indicates the formation of Si(111)‐like facets.