Atomic step structure and chemical termination of Si(lll) surfaces after wet chemical treatments which precede the various oxidation, deposition or growth processes for the patterning of microelectronic devices—i.e. various cleaning, HF or NH4F etching, and H2O rinsing steps—are investigated by scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and thermodesorption spectroscopy (TDS) after sample transfer into ultrahigh vacuum: Concentrated HF selectively removes surface oxide and, thus, chemically prepares the initially burried, isotropically rough Si/SiO2 interface. A surface termination by mono–, di–, and trihydrides results from the differently coordinated interface defect sites and can be distinguished by distinct characteristic H2 desorption peaks in TDS. NH4F, on the contrary, attacks the bulk silicon anisotropically to form large, ideal monohydrideterminated (111) terraces with triangular etch pits on precisely (111)–oriented samples or to result in a rapid silicon removal by a flow of the preferentially etched atomic steps on samples with an initial miscut. At weak miscut the etching anisotropy dictates a flow of (kinked) steps made up of straight monohydride–type step portions perpendicular to <211> while at strong miscut the miscut–promoted step type survives due to strong stepstep interaction and even a flow of (unstable) dihydride–type steps may be observed if the sample surface is tilted towards <211>. During the experiments the sample cleanliness (carbon–hydrides) is monitored by AES, comparing various wet chemical and gaseous phase ex situ cleaning and etching techniques.